RPi OSW-Picked

You've picked an OS!

What's Linux?

As you don't know what Linux is, it's preferable to skip over to the Raspbian section as thats the canonical Linux version used on the PI (but Linux isn't the only choice).

IWannaWindows

Oh, I see how it is. but wIndows only supports x86 CPU architectures, so it wont run on ARM. And neither would its applications. windows isnt open source (obviously) so there is no way to port it to another architectue, but reactos is, so consider editing ReactOS and make it ARM compatible. (If you know what to do, that is. :) It would be awesome to the community!)
But do understand that even if ReactOS itself would work, you could run none of the x86 Windows applications on it.
If you have heard that "windows8 (actually windows RT, the ARM variety) runs on ARM", that wont help you either, as Windows 8 only runs on newer multi-core ARM versions, and besides Microsoft demands technical measures that would make it impossible to run anything other than windows 8 on the hardware, and not only is that against the spirit or the raspberry PI, it is also technically impossible to implement such restrictions on the PI. In short Microsoft would never agree to support the raspberry PI. You should also realize that even when Win8 would somehow would run on the PI you could still not run x86 (intel) windows applications on it, and win8 would be far too resource demanding even for a 512MB PI. As for windows CE (and its predecessors) the use for it would be very limited, and microsoft demands licenses for its use too, its not something you can easily and legally port to the PI. And is not really Windows "as we know it".

Raspbian

Hello, newcomer (to Linux and/or the Raspberry Pi). To get Raspbian, the official OS for the Raspberry Pi, click here. Raspbian is a specialized version of Linux just for the Raspberry Pi!

Nobuntu

Because of Canonical's stubborn decision, they have decided to not support ARM 6 and below. We'd recommend either Raspbian or Debian, as Ubuntu is a derivative of Debian.

Fedora or Red Hat

Shoot! Red Hat is not making an ARM version available! Please try the default distro.

ArchLinux

Hmm, congrats! Arch Linux IS supported by the RasPi Foundation! Please click here for an Arch Linux download for the RasPi.

Debian

Ok. Good for you. Please click here for an appropriate Debian download for the RasPi.

Puppy linux

Often regarded as the ultimate Linux distro for "small systems", there is now an experimental version of Puppy Linux for the PI called "Squeezed Arm Puppy". Its especially suitable for people who never used anything but Windows, as its made to somewhat resemble and work like windows, but its still Linux. It comes with many extras.

AROS

AROS research operating System. A remake of the old Amiga OS

Android

Googles OS for mobile systems, is in development in the form of Android 4.0 ("Ice Cream Sandwich"). It's not out yet.

Firefox OS

An OS based on the code behind the browser Firefox, also in development.

WebOS

Based on the former OS for Palm, later acquired by HP, in the form of "OpenWebOS" an open source version is in development

Plan 9 from Bell Labs

Plan 9 is a very experimental OS, which is loosely based on the original Unix taken to its extremes, only suitable (written for) programmers, at this time.

RISC OS

easily the canonical OS for ARM based computers, first developed for the Acorn Archimedes, the computer that should be te successor of the BBC Beeb, the computer that inspired the PI (its why the two versions of the PI are called the "model A" and "Model B", just like the Beeb from yesteryears. Very fast and smooth running on the PI. and there is even a version of firefox running on it, and, not to forget it runs BBC BASIC!

BSD

Berkley Software Distribution. Although not legally Unix FreeBSD and NETBSD are two very Unix like OS's which are well established. especially useful for very security minded users. Rumors are that even the NSA is using it.

Others

Not exactly OS's in a technical sense, but there are also specific software sets that can be put on a SD-Card for specific applications. Just to mention a few, there is XMBC which is a dedicated media player, and Instant Kiosk which is an instant ethernet browser (kiosk) system.

Other similar packages are IPFire, OpenELEC, Raspbmc, XBMC and XBian