RPi Expansion Boards
Hardware & Peripherals:
Hardware - detailed information about the Raspberry Pi boards.
Hardware History - guide to the Raspberry Pi models.
Low-level Peripherals - using the GPIO and other connectors.
Expansion Boards - GPIO plug-in boards providing additional functionality.
Screens - attaching a screen to the Raspberry Pi.
Cases - lots of nice cases to protect the Raspberry Pi.
Other Peripherals - all sorts of peripherals used with the Raspberry Pi.
On the Raspberry Pi, there are several connections which can be used for expansion:
- The Rpi GPIO (General Purpose Input/Output) pins are exposed, that means that expansion boards are able to talk directly to the CPU.
- The DSI connector will allow low-level interfacing with LCDs and other displays.
- The CSI connector will allow camera modules to be added in future.
Expansion boards by functions
Alim / Power Supply and Control
Meltwater's Switched Mode Power Supply Kit
This is a DC to DC switched mode power supply, another great add on board in Meltwater's range. It comes as a kit of parts, ready to be soldered together. This can supply a 5 volt output to a Raspberry Pi from an input of anywhere from 7 to 40 volts!
Testing has shown that the unit is around 80% efficient, where a comparable linear regulator (such as LM350T) was measured under 50% (based on a quick test).
Pi Supply Switch
The Pi Supply Switch is an "intelligent" power management device for the Raspberry Pi made by Pi Supply. This add on board has hard on/off switches as well as a soft shutdown switch, which initiates a safe shutdown using a simple Python script. It allows you to cycle power on the Raspberry Pi without plugging and unplugging either your "wall-wart" power supply, or the micro-USB plug input to your Pi. It includes an LED status indicator, and 3 mounting holes for easy use in any project you have in mind. It needs only one GPIO pin to operate, with the option of using a second GPIO pin to add the safe shutdown functionality.
It is an Open Source project, and comes with free stickers and a free noodle USB cable to connect to your Pi.
The Pi Supply was funded using the crowdfunding site, Kickstarter.
RPI Alim is a inexpensive board that gives the possibility of powering the Raspberry with a 7V to 15V DC power supply or battery.
The RemotePi Board is an intelligent infrared remote controlled power switch add-on board for the Raspberry Pi. It allows to switch power on and off using any button (configurable in learning mode) of an existing standard IR remote. Power is only cut after notifying the OS and giving it time to shut-down. It is mainly intended to remote control (using LIRC) and power off/on a mediacenter system. i.e OpenELEC, Raspbmc, XBian, RasPlex, Raspbian. The board is compatible to simple GPIO IR receiver and piggy backs onto the Raspberry Pi, no soldering required. For more information click here.
3V3 DC-DC Switched Mode Regulator
The Raspberry Pi uses a linear regulator to convert the input 5V to 3V3. This converter can be replaced with a more efficient switching mode to reduce the power needed. This can be useful when running the Pi on battery. More information and a ready-to-buy kit be found here
Protect & leds
Buffer Board How to make a Pi Buffer on Veroboard. Each of the 14 outputs can drive up to 500mA using a Darlington array.
EASY GPIO is an GPIO expansion board.(GPIO are protected by diode.)
- 3 leds + 3 button
- 6 GPIO ( Protected by diode)
- Serial port ( Protected by diode) 3.3V ( compatible 5 V)
- I2C ( Protected by diode) I2C 3.3V
- SPI ( Protected by diode) SPI 3.3V
BerryClip 6 LED Buzzer Board
The BerryClip is a small board providing 6 coloured LEDs, 1 buzzer and 1 switch. It provides an easy and quick introduction to GPIO experimenting. The board plugs directly onto the Raspberry Pi GPIO connector and there are lots of Python example scripts available.
For a more detailed description see: BerryClip on Raspberrypi-spy.co.uk
LedBorg is an ultra bright RGB LED add on board for your Raspberry Pi. Capable of 27 colour combinations and controllable from either a GUI or the command line, it can also perform patterns.
Small board which connects directly to the GPIO header, Light up your case to suit your mood or display status information.
Meltwater's RGB LED Board
As well as a great blog, Meltwater also has a selection of add on boards, including a RGB LED board. This includes 5 bright RGB LEDs, and uses just 8 GPIO pins. The kit includes a pre-prepared strip-board, all the necessary components and cables, and a very complete full-colour manual with lots of code examples.
This is a great kit for learning how to use the GPIO on the Raspberry Pi, and make some pretty colours while you are at it!
These boards are also available from the Pi Supply shop.
GPIO isolated expansion/RTC
UT Electronics is developing a expansion board containing a fully isolated 16-I/O I2C GPIO expander and RTC, with extensive options for further expansion. C and Python libraries will be available at launch. The board will probably be sold as a kit, with SMT components already in place.
Update: According to their website, the realities of life prevent them from bringing the product to market. They are hoping someone will pick up the ball. They have released the PCB layout and all documentation for free on their website.
LEDs + microswitch
AFLEX Robotic Board
AFLEX Robotic Board is dual motor control and data acquisition board with I2C and Serial interfaces. The board provides 2 motor control drivers with max current up to 3.5A, 8-bits data port with each pin setup as digital input /output or analog input, 10-bits ADC with up to 5 analog channels, 4 inputs for the sensor connections, infrared (IR) remote control. The board provides direct connection to Raspberry Pi.
The BrickPi from Dexter Industries helps you connect LEGO® Mindstorms sensors, motors, and parts to easily turn your credit card size computer into a powerful robot.Some highlights of the product include:
- Connect up to 4 NXT or EV3 motors.
- Connect up to 5 NXT sensors, both digital and analog.
- The BrickPi power supply can power the Raspberry Pi directly.
- Attach LEGO® Technic bricks to the BrickPi case to make awesome robots!
Mobile Robot Chassis controlled from RPi
Mobile Robot Chassis controlled from RPi is a smart robot platform with embedded robot controller. NO SOLDERING NEEDED. The controller board includes the preprogrammed microcontroller, 4 DC motor drivers and circuitry to read data from sensors. The preloaded software provides the DC motors and Servo motors PWM control, sensor data reading and communication with RPi by I2C interface or wireless channel.
PicoBorg is an inexpensive add on board for controlling robotics such as small motors, fans, solenoids, LEDs and the like from the Raspberry Pi GPIO pins. It consists of 4 low side drivers and 4 diodes, and is hardware selectable for either using the Pi PWM output or only GPIO pins.
PicoBorg Reverse is an advanced motor control board for Raspberry Pi based robotics projects. It can control small and large motors alike (up to 25 V and 5 A per channel), either driving two DC motors (forward and reverse) at controllable speeds or a single DC motor or stepper at up to 10 A (4, 5, and 6 wire steppers supported). Capable of daisy-chaining multiple boards, it can allow over 200 motors to be controlled from a single Raspberry Pi.
It is the motor control board that powers DoodleBorg.
PiServoController board from OpenElectrons.com fits over the GPIO pins of Raspberry Pi, and it can control upto 6 servos from Raspberry Pi. It has i2c interface to control i2c devices. This board provides power stabilization for Raspberry Pi, so that any power fluctuations won't cause RPi to reset. It also provides signal buffering so that you can connect higher voltage servos to the board. For Servo control, it uses Open Source Software, ServoBlaster. Users can program the servo movements in Python.
RoboPi Advanced Robot Controller for the Raspberry Pi
RoboPi Advanced Robot Controller Raspberry Pi expansion board from Mikronauts adds an eight-core 32-bit microcontroller running at 100Mhz to the Raspberry Pi in order to off-load hard real time I/O and allow more precise timing than Linux running on the Pi allows.
- 8 channel 12 bit Analog to digital converter
- 24 digital I/O's each of which can be independently confidugred as digital input, digital output, PWM output, servo output, Ping sensor input
- 10 pin expansion headers for the 8 core microcontroller (Parallax Propeller)
- i2c expansion headers for both RoboPi and the Raspberry Pi
- easy to use Wiring-like API for controlling RoboPi fro the Raspberry Pi with C / C++ (Python API coming soon)
Robot Controller with direct connection to RPi
Robot Controller with direct connection to RPi is a versalite Mobile Robot Chassis controller board with I2C interface and wireless communication channel. The controller board includes the preprogrammed microcontroller and provides the motor drivers to control 4 DC motors, 2 speed encoder channels, ultrasonic distance measurement channel, infrared obstacle avoidance sensor and 5 analog channels for your sensor connections (line following sensors or obstacle avoidance sensors). The board also provides 2 Servo motor controls to create the robot pan/tilt head.
RPi Servo board
The Raspberry Pi Servo board by adent is a small expansion board for connecting up to eight RC servos and/or an ESC (Electronic Speed Controller) to the RPi. It has a pass through for the GPIO port and it uses the serial line in it for commands. The interface is a simple ASCII-protocol via the serial port. It also has five own GPIO-pins that is not supported by card firmware yet. Firmware is released as GPL. Can power RPi from ESC or external source.
Youtube clip of first prototype. Working on an alternate firmware to connect DHT22/11 humidity/temperature sensors to the board instead.
RPi SOFTWARE Servo board
The Raspberry Pi SOFTWARE Servo board by adent is a small expansion board for connecting up to eight RC servos and/or an ESC (Electronic Speed Controller) to the RPi. It has a pass through for the GPIO port. Can power RPi from ESC or external source. It needs a kernel module to work. The card only buffers the GPIO of the RPi and provides convenient connectors. Since there was room on the PCB, the serialport, GND and 3V3 is broken out.
XLoBorg is a 3 axis accelerometer and 3 axis magnetometer which can be used as a compass and would be especially useful in UAV/drones. It plugs directly into the Raspberry Pi and requires no additional connections.
Multi-Pi is motor* drive extention shield for raspberry-pi and can add many sensor, clcd, glcd etc.. with motor dirve on the shield. motor* : dc motor, bldc(add resister), stepping. in addition, Only sn754410(or SN754410NE)motor drive if not use this can be used as a various raspberrypi extention shield. we has been post on the www.rasplay.org: our website
- RC Car for Raspberrypi Project post click
- Uart connect for Raspberrypi post click
- USB HUB for Raspberrypi post click
- Pi monitoring GLCD post click
- ETC ... post click
PiPan is a Pan/Tilt Kit for PiCamera designed by OpenElectrons.com. Pi-Pan provides Pan and tilt movements for your Raspberry Pi Camera, using two servos for X and Y axis movements. The kit includes servos, mounting hardware and PiServoController board to control servo movements. Users can program the servo control/camera movements, etc. in Python. There also is an addon light that can be fitted in front of the PiCamera to illuminate subjects, PiLight.
RPIDOM is an inexpensive expansion board dedicated to home automation. It provides a 1wire interface, 2 teleinformation interfaces (french power meters), an RTC and a 5V connector.
Heber Ltd X10i integrates real time I/O into a RPi control system. A secure real-time controller that permits control over multiple and different types of inputs and outputs, and uses the USB 2.0 communication standard. It is an all in one plug and play board for developing LED drivers, control, sensing of different switches, and driving stepper motors. Exclusive features include a real time clock, battery backed static ram, a Dallas chip (for unique board identification), 2 serial communication ports, a powerful audio amp and a independent dedicated on board processor.
Heber now offers drivers in C/C++ and Linux to make it even easier for programmers, inventors and developers to connect with the outside world using X10i and their Raspberry Pi.
Single and 8 Channel 1-Wire Masters from Sheepwalk Electronics
Sheepwalk Electronics have developed a series of expansion boards for the Raspberry Pi to interface your Raspberry Pi to a Dallas/Maxim 1-Wire network. The range includes a simple passive "master" (called the RPI1) to allow easy use of the kernel 1-Wire driver, a single channel I2C master based around the DS2482-100 (called the RPI2) and an 8 channel I2C master based around the DS2482-800 (the RPI3). The modules are now all available for sale on our website. We also sell a range 1-Wire slave boards including temperature and humidity sensors and I/O boards.
Pi232 RS232 board
Pi232 is an RS232 expansion board that plugs onto GPIO connector.
Pi-Wire Homelabs are developing options for connection of a 1-wire bus to the RaspberryPi. Boards include SPI breakout connections for future boards.
BitWizard has a Raspberrypi serial breakout board. It breaks out four serial buses from the GPIO port.
BitWizard then has a series SPI (or I2C) expansion boards that can be daisy-chained off the SPI (or I2C) buses of the expansion board. A virtually unlimited number of expansion boards can be connected. Expansion boards are available that drive a simple 16x2 text LCD, more general purpose IOs, servos, temperature sensors, relays, optocouplers etc.
RPi RS232 Breakout Board
RS232 to TTL converter
USB - RS232 TTL / I2C translator
set01_05 has three functions : 1st) translating USB to TTL Tx and Rx to be connected to 3Bpi, 2nd) converting USB to I2C and 3rd) supplying voltage to next board (it is not for 3Bpi) if jumper is between positions 1 and 2.
Wolfson sound card
The equivalent of a high end sound card for the PI. Features:
- 3.5 mm jack (4-pole) for a headset/boom mic combination for gaming or VoIP applications
- 3.5 mm jack for microphone input
- On-board MEMS microphone
- 3.5 mm jack stereo line input for connection to devices such as digital audio players (iPod etc.) or mobile phones
- 3.5 mm jack stereo line output for connection to devices such as external stereo amplifiers or powered speakers
- Phono jack for electrical (not optical) S/PDIF digital input
- On-board class D power amplifier for external speakers, with connection to external power source if needed.
- Small pin header for extra functions if they are low cost, already on-chip, and don't require any further components
- Raspberry Pi form factor
A high-end 24bit/192kHz DAC that connects to the I2S header. The board has RCA and phono jack line-level outputs with 2Vrms output, but does not need an external power supply.
An S/PDIF output card for the Raspberry Pi. Features an optical and an isolated electrical output
Arduino Shield and Raspberry Pi
a little board with Arduino socket and Raspberry Pi socket you can power the boards with 12Volt or 5Volt http://www.progetti-hw-sw.it/raspberry.htm
“À la mode” is an Arduino clone specifically designed to interface with the Raspberry Pi. You can of course connect a standard Arduino to a Pi USB port using a cord, but when you want a turnkey solution, how about an Arduino compatible “plate” (shield) that fits right on top of the Raspberry Pi with direct access to GPIO port? The Linux side handles all of the displays, human interfaces, and number crunching. The low power Arduino compatible “à la mode” board handles sensors, servos/motors, and provides a highly accurate real time clock.
The ArduiPi is a board that brings Arduino world to Raspberry Pi and vice-versa. It is totally open source Hardware and Software.
The features are:
- Power the Arduino side with Raspberry power or with external power.
- Selectable Arduino side power with 5V or 3.3V
- FTDI cable connector to be able to program Arduino independently leaving RX/TX of arduino and Raspberry free.
- ICSP connector.
- Power level shifter between Pi and Arduino for I2C, SPI, Serial, and all Pi V1 and V2 GPIO pins.
- Raspberry Pi board Revision 2 with four new GPIO
- I2C Bus available from Arduino AND/OR Raspberry Pi
- I2C to 1-Wire controller on the board providing fully OWFS (or other) support for Raspberry Pi OR Arduino with dedicated library
- Feature to connect I2C between Arduino And Raspberry Pi (Arduino will become a I2C slave from Raspberry Pi)
- Feature to connect SPI between Arduino and Raspberry Pi (Arduino will become a SPI slave from Raspberry)
- Feature to connect the Arduino UART to Raspberry Pi Serial to do communication
- Availability to use the French dedicated Téléinformation monitoring electricity signal issued from the main power meter to Arduino or Raspberry Pi
- Grove connector on board for I2C and Serial connection for Raspberry and Arduino
- Sized place to plug aAdafruit I2C or SPI Oled breakout board that can be driven by Arduino or Raspberry Pi in SPI or I2C
- Sized place to plug a NRF24L01 Wireless breakout board. Breakout board fitting this format will be available for RFM12B, RFM69W, SD card and SPI flash memory. This will let you use the majority of RF module used today
- Sized place for 3 x 1-Wire devices on board.
- Sized place for using PWM devices (switchable with on board power or external power)
- Two free pins available on level shifter chip for custom usage.
Raspberry Pi to Arduino Shields Connection Bridge
Raspberry Pi to Arduino Shields Connection Bridge allows you to use any of the shields, boards and modules designed for Arduino in Raspberry Pi. It includes also the possibility of connecting digital and analog sensors, using the same pinout of Arduino but with the power and capabilities of Raspberry. Using arduPi library, you will be able to make programs in Raspberry with the same code used in Arduino. To do so, Cooking Hacks Team have implemented conversion functions so that you can control in the same way as in Arduino all the I/O interfaces: i2C, SPI, UART, analog, digital, in Raspberry Pi.
Paper-Duino-Pi is an Arduino shield for the Raspberry Pi. Due to the fact that it is designed as Paper-PCB it is easy to create and one doesn't need a printed circuit board. All components are cheap standard electronic parts. A video is found here.
Youtube video showing prototype board
PONTE initial post An arduino shield bridge from Omer and SKPang
RPi-ShieldBridge An Arduino Shield Bridge with connector for a 2.8" TFT-Display with Touchpanel.
RPI RTC is a inexpensive Real Time Clock (RTC) board.
On 2nd January 2012, CJE/4D announced that they have a real-time clock (RTC) expansion board in the works: 
AbioRTC is an inexpensive real-time clock (RTC) expansion board.
The AdvaBoard RPi1 contains an interface for TFT displays (3.2" - 7.0") incl. touchscreen. For details, see [#advaboard_rpi1 below], or visit the websites www.advaboard.de or www.advamation.com/products/embeddedpc/advaboard.html.
The 'Tontec' 2.4-inch TFT LCD 240x320 RGB Pixels touchscreen is a small board that plugs into J1. The touch part uses a TSC2003 chip on the I2C bus; the screen data goes trough the SPI bus and uses a couple of GPIO. There are pins to extend SPI & I2C bus and a GPIO extender LP3943 , '16-LED Fun Light Driver' http://www.ti.com/lit/ds/snvs256b/snvs256b.pdf). See http://helloraspberrypi.blogspot.be/2014/01/setup-tontec-24-inch-tft-lcd-240x320.html for the display and http://www.raspberrypi.org/forum/viewtopic.php?f=91&t=64414 for a discussion about the 'touch' part. (See MZTX-PI-EXT)
The AdvaBoard RPi1 contains:
- TFT-display-interface (3.2" 320x240, 5" 800x480, 7" 800x480)
- touchscreen interface
- real-time-clock (RTC)
- CPLD + a microcontroller
- all I/Os are 5V-tolerant, except I2C
- analog-I/Os (4* 16bit inputs, 2* 8bit outputs)
- separate connectors for all interfaces (I2C, SPI, digital/analog-I/O, RS-232, RS-485 incl. pluggable terminator)
- power-management: battery-backed RPC/I2C-interfaces, power-down, power-up on time or event, separately powered peripherals
- JTAG-programmer for Xilinx CPLDs/FPGAs, C2-programmer for SiLabs C8051F*
- software (C + Python 2.6/3.x libraries, free example programs)
Details can be found at www.advamation.com/products/embeddedpc/advaboard.html
- direct I2C connector
- 5-way joystick
with driver support. More information: Technical Specifications.
www.pic-pi.com python libraries and hardware for RS232, RS485, Real Time Clock, 2x Analog outputs, 2x 240vac 10 A Relays, 22x Individually configurable Analog inputs of digital IO
 PiCAN plugs onto GPIO pins and contains:
- CAN controller and transceiver (2515 based). DB9 connection to vehicle OBDII connector, pins for a robot Controller Area Network.
- 5V 2amp regulator for battery powered systems (6 to 24V)
- EM406 GPS module connector
- 3 GPIO buttons
- 4 LEDS
Buzzberry Buzz-Board Interface
Buzzberry is an interface board enabling expansion using the Buzz-Board modular system designed primarily for educational use.
- main page: RPi Gertboard
This board is a buffer between your Rpi and the real world, use it to flash LEDs on and off, drive motors, run sensors and all that other fun stuff.
GELI 'jelly'(GPIO Experimenter and Lab Interface Board)
Raspberry Geli Board Extends the GPIO and provides I2C, D2A/A2D, DC Motor(L298), RS232, Realtime Clock (DS1307), 1-wire and prototyping area on 150 * 100mmm PCB.
- Directly access Grove Analog, digital,serial and I2C sensors from Raspberry Pi.
- Libraries available in C and Python
- Modular plug and play modules, no more headaches for wiring the sensors. All modules are connect using a simple 4 wire cable.
picru.st is a very compact and cheap breakout board for your Raspberry Pi that allows easy access to the Pi's pins, grouping them into a logical order and adding clear labeling. Small and simple. Now available to purchase in kit form on Kickstarter here - Pi Crust Kickstarter
Pi MiniLab - Education / Experimental Board
Pi MiniLab Education / Experimental Board is a small size RPi add-ons board. The board includes the circuitry for the experiments with analog signals, sensor connections and circuitry to control the external devices as the relays, motors, high current LEDs. Key Features: 12-bit ADC with 5 analog channels, 12-bit DAC, 4 analog amplifiers, analog comparator, frequency generator, 1-Wire port, on-boar potentiometer, RGB LED, 4 high power outputs, circuitry for LCD 20x4 or 16x2, voltage regulator. Board size - 2.1"x1.6" (53mm x 41mm).
EZasPi prototyping board
Mikronauts EZasPi advanced prototyping board that stacks on top of the Raspberry Pi and was designed by Mikronauts to make prototyping easier for educational users, and making one-off expansion circuits easier for everyone. EZasPi also has a four pin I2C expansion header, and a five pin COM connector for future expansion.
EZasPi comes as a kit which includes an extra tall stacking header and mounting hardware that secures the board to the Raspberry Pi Rev.2 test mounting holes - keeping the board securely mounted on the Pi.
Multiple EZasPi boards can be stacked on a single Raspberry Pi without the need for a bus or rack solution. The board has an extra large prototyping area that is silk screened to clearly indicate signal busses. For example, you can stack an EZasPi board between your Raspberry Pi and a RoboPi robot controller to add additional circuits to the Raspberry Pi.
You can make a Pi Sandwich by stacking an EZasPi on top of your Raspberry Pi, and an EZasPi (B) under your Pi - with the mounting hardware securing your boards together.
EZasPi (B) prototyping board
Mikronauts EZasPi (B) advanced prototyping board from Mikronauts is first prototyping board that stacks UNDER the rev.2 Raspberry Pi. EZasPi was designed to make prototyping easier for educational users, and making one-off expansion circuits easier for everyone. EZasPi (B) also has a four pin I2C expansion header for future expansion.
EZasPi (B) comes as a kit which includes an extra tall stacking header and mounting hardware that secures the board to the Raspberry Pi Rev.2 test mounting holes - keeping the board securely mounted on the Pi.
Multiple EZasPi (B) boards can be stacked beneath a single Raspberry Pi without the need for a bus or rack solution. The board has an extra large prototyping area that is silk screened to clearly indicate signal busses.
You can make a Pi Sandwich by stacking an EZasPi on top of your Raspberry Pi, and an EZasPi (B) under your Pi - with the mounting hardware securing your boards together.
PiPLUS Multifunction I/O Extension Board
PiPLUS Multifunction I/O Extension Board is designed as versatile, general-purpose data acquisition and control module. Key Features: Battery-backed Real Time Clock; 3.5A DC motor driver chip; 8-bits digital I/O Data Port; 10-bits ADC with 4 analog channels; DAC; 4 high power outputs; 4 PWM outputs; IR Remote Control input; 2 user's buttons; AVR ATMEGA328P microcontroller with a preloaded Arduino bootloader and Arduino Shields connectors; 3V - 5V buffers for I2C, SPI and Serial interfaces; Power supply with 5V and 3V regulators.
PiPLUS board also works as a bridge between Raspberry Pi and ARDUINO and provides a direct connection to Arduino Shields. You can use the standard ARDUINO environment and graphical interface for Windows or Linux for software development and ATMEGA328P microcontroller programming.
Not yet tested, comments welcome Project files for Eagle here:  Schematic and Board images:   This board uses Microchip 16-pin GPIO expanders driven by SPI to provide up to 256 additional GPIO pins at a very low cost. Optionally, it can supply power for the board and the Raspberry PI.
The board can be made in a stackable manner allowing you to also place additional peripherals on the GPIO connector. It makes non-exclusive use of all four SPI pins as well as exclusive use of the CE0 Pin.
If you install (and power) the power supply components, the board will supply up to 1A at 5VDC. This is not enough to power all GPIO pins on a fully populated board, but in most applications should be sufficient to power the RPI and a reasonable number of GPIOs. An external power supply should be used for higher-power applications. This board should never be powered from the RPI as even one of the chips can draw more than the RPI can provide.
The power supply is quite flexible and will accept anything from 8-25VAC or 8-42VDC. It is based on a Recom module which provides a switching power supply in a form factor that is a drop-in replacement for 7805 series TO-220 linear regulators.
The 8 chips can be had for about $4@ or less and the power supply components total about $15. A fully populated board is about $49+PCB, $31+PCB for 64 GPIO pins.
You can have the PCB made at a variety of fabricators.  does very nice work and is hobbyist friendly at very reasonable prices.
PiWeather - Raspberry Pi Weather Station Board
PiWeather RPi Weather Station Board is a small size RPi add-ons board. The board provides the temperature, pressure, humidity monitoring and controls the external devices (relays, high current LEDs ...).
Key Features: Temperature, pressure and humidity sensors on the board; Two 1-Wire ports to connect the external temperature sensors DS18B20; Four high current outputs; Battery-backed Real Time Clock and Calendar; Small size - 2.1"x1.6" (53mm x 41mm).
Quick2Wire GPIO Interface Board Kit
Quick2Wire GPIO board and experimenters kit is an extension board for the Raspberry Pi that has level converters to allow the driving of TTL and real world devices such as LED's and switches. It will also provide an experimenters kit, a software framework and a number of tutorials. The Quick2Wire interface board is also available from Pi Supply.
Quick2Wire I2C PCF8591 Analogue Board
The Quick2Wire PCF8591 I2C Analogue Board has four 8-bit Analogue Inputs and one 8-bit Analogue Output. A simple Python API allows you to control the on-board PCF8591 chip and access all of its features for use with the Raspberry Pi. You can use the boards to measure four voltages and control one output voltage! - You can purchase from Pi Supply
Quick2Wire I2C MCP23017 Port Expander Board Kit
The Quick2Wire Port Expander board for the Raspberry Pi, gives you an extra 16 GPIO pins which can be used for digital input or output. You’ll find code in the Quick2Wire Library to drive it from Python, along with examples of use. - You can purchase from Pi Supply
Raspberry Pi IO Interface
forum thread showing 'paper mockup'. Another buffered IO expander.
Relay board and Raspberry Pi GPIO
SainSmart X100 Expansion Board
SainSmart X100 Expansion Board SainSmart offers the X100 expansion board "designed for ease of use", intended to make the RPi easier to use by providing various interface ports, such as a RS232 port, a HDMI to VGA converter (by using an included HDMI to HDMI male adapter, enabling the use of the VGA port; otherwise it is a HDMI input), a 3-port USB hub (one of the X100's USB ports connects to the upper USB port on the RPi itself, using an included USB to USB male adapter, thus providing power for the three other USB ports on the X100 board), SD and microSD memory card readers, an 8-channel Darlington driver and a GPIO port. The board also has a battery-powered RTC, and a variable (6-23V, nominally 12V) power port for a corded power supply. The X100 also has a reset switch, but the reset pin header (included with the X100) must be soldered onto the RPi itself.
Slice of Pi
Slice of Pi Ciseco have produced a convienient small break out board for the Raspberry PI. XBee style connector for XRF / XBee / RN-XV / XBT / RF-BEE etc. Has the SPI and I2C pins on one standard 8 way header and the Raspberry PI's 8 general purpose I/O pins on another for easy access. Comes as a kit of parts that needs to be soldered. Construction is very easy and suitable for children with basic soldering skills.
Slice of PI/O is a variation of the above produced by Ciseco based upon Nathan Chantrell's MCP23017/Slice of Pi "project". The MCP23017 may be powered from either the Pi's 3.3V/5.0V GPIO pins or, with care, an external psu. The use of a 5.0V power source means that the MCP23017's 16 (2 x 8) GPIO's can interface directly with 5V logic families etc. However, the Pi's own GPIO's, also available, will still need 3.3V - 5.0V level shifters. Trevor Gowen's experimental LED BarGraph "board" has been "piggy-backed" onto one of these.
A GPIO header replicator board for your Raspberry Pi. Triple the Raspberry Pi GPIO header.
XBee ZB SMT Backpack
The Raspberry Pi XBee ZB SMT Backpack by bootc is a carrier board for Digi XBee ZB SMT modules. These are different to the normal XBee modules because they are surface mount and additionally can interface to a host using SPI as well as serial. The backpack also features 3 LEDs for status signals as well as two FTDI headers (one for the RPi and one for the XBee), as well as being able to host a 'normal' XBee module (though of course you can't use SPI with those).
Raspy Juice Exp Board
Raspy Juice by 2-Watt Elements is an experimental board that supplies the Raspberry Pi with +5V through the GPIO header. With its wide input voltage range (6~23V), the buck regulator can be powered by batteries, 12V Adapters, solar, etc. Other features are the RS232 level console port, PCF8523 RTC, and an expansion ATmega168A microcontroller as an I2C slave. This latter MCU has interfaces of an RS485 port, an additional RS232 port, 4 RC-servo ports and unused-pins expansion header. The MCU firmware is updateable through the I2C interface. Hardware details are in http://code.google.com/p/raspy-juice/wiki/1_Hardware_Description
pyMCU is a python controlled microcontroller that works with the Raspberry Pi, connects to the USB port and uses the pyMCU python module and pyserial to control the microcontroller in your python script. You can communicate with i2c, spi, serial, and one wire devices, software configurable pulse in / out functions, has 13 Digital IO Pins, 6 10-bit Analog Pins, 5 10-Bit PWM Pins, blink LEDs, control servos and motors, read various sensors, built-in functions for sound generation: arbitrary frequency generator, misc. sound / tone generator, DTMF generator. PyBootloader - python script for uploading new firmware versions or your own microcontroller code. Check out some of the Tutorials and Examples
RPi CC1101-OneWire-Clock extension
The RPi-COC board from busware.de adds Onewire, a Sub-1 GHz RF Transceiver, RTC and an EEPROM mainly for smart home automation. The Onewire RJ11 port is driven by a DS2482 chip supported by 5V levelshifter and strong pullup. The DS1339-RTC is powered by CR2032 battery or on-board 60mF supercap. The 2Kbit EEPROM (at24c02) is mainly to hold board specific config data. The CC1101 Sub-1 GHz RF Transceiver is controlled by an atmega1284 to pre-process air data. It is connected to RPis UART and contains a avr109-bootloader for firmware updates. The popular culfw-firmware lets you interact with FS20, Homematic, Intertechno or similar actors and sensors - easily using FHEM-home automation server. In addition a secured 5V power socket for external power supplies is provided.
RFM12Pi - Wireless OpenEnergyMonitor Base-station
Add's RFM12B wireless (868/433/912Mhz) RF transceiver capability to the Raspberry Pi. The RFM12Pi decodes RFM12B packets from OpenEnergyMonitor sensor modules or JeeNodes and puts the data on the Raspberry Pi's internal serial UART. Designed to be used with emoncms running on a Raspberry Pi web-server to make a low power energy, temperature and other environmental data logging and visualisation server. Developed as part of the OpenEnergyMonitor project. For more info see introductory blog post and overview documentation page. The RFM12Pi can be purchased pre-assembled and ready to go from the OpenEnergyMonitor shop
3Bpi - Three Branch Pi - Level translator board for Raspberry Pi
3Bpi, aka set05_08, translates bidirectionally inputs/outputs of the Raspberry Pi's GPIO from 3.3V to 5V TTL levels. It has three external ports to connect electronics.cat open source hardware boards. This board takes advantage of P5's Raspberry Pi, with access to a second I2C channel (I2C_0, it can be used without camera connected) and CTS and RTS for hardware flow control in serial communications. Two of 3Bpi's ports, P3 and P4, can be used as 8 bit input/output or I2C and/or SPI ports. One port, P2, is thought for serial communications and Raspberry Pi can be set as master or slave setting the jumpers properly.
AbioWire is a 1-Wire adapter providing 17 1-Wire channels with ESD protection, 3 1-Wire controller chips, and a real-time clock (RTC).
AbioCard model A
AbioCard model A expansion board featuring a real-time clock (RTC), 8 bidirectional I/O lines, 8 12-bit ADC inputs, 16 PWM outputs at 97 kHz with 8-bit resolution.
AbioCard model B
AbioCard model B expansion board featuring a real-time clock (RTC), 8 bidirectional I/O lines, 8 12-bit ADC inputs, 16 PWM outputs at 40-1000 Hz with 12-bit resolution.
Rs-Pi P-HAB module ( High-Altitude Balloon)
Rs-Pi P-HAB expansion board featuring a ublox MAX 7 GPS module, FM transmitter, Temp, Humidity, Pressure, Altitude sensors), This project use balloons for exploring the edge of space. collect temperature , Humidity, Altitude, Pressure data send back through FM Transmitter and save all log to SD card, also take a photo every 30sec save to SD card. The project use 1. ublox-7Max GPS module 2. Radiometrix NTX2B 434.xxx Mhz 10mW modules. 3. TMP102 temp sensor to monitor Raspberry Pi temperature 4. BMP085 Barometric Pressure/Temp/Altitude Sensor 5. DHT22 / AM2302 Digital Temp & Humidity Sensor 6. DS18b20 Temperature sensor 7. TRACOPOWER 5V & 3.3V (TSR-1-2433 & TSR-1-2450) DC/DC converter up to 96% efficiency - No heat-sink required ( for longer battery life) 8. Raspberry Pi Camera kit ( take photos save in SD card)
The Guzunty Pi adds open source programmable hardware to the Raspberry Pi at an amazingly low price. The design contains a 72 macrocell programmable logic device, so the board functions like a 'virtual breadboard'. This board is ideal for those with many different projects in mind, because it is so flexible. It can be a simple IO expander, a 7 segment LED driver, provide servo and regular PWM outputs, or anything you can dream up that will fit into 1600 logic gates. The device is '5 volt tolerant' and so can be used to safely interface 5v logic devices (such as Arduino) to the 3.3v Raspberry Pi. The GitHub community site contains many ready made cores that you can download and use directly. You can design your own custom cores using industry standard languages such as VHDL or Verilog.
Expansion boards by manufactuer
This section has been moved to a separate page. See RPi Expansion Boards: Expansion boards by manufactuer