Open Wrt is a Linux distribution for embedded devices, especially wireless routers.
Instead of trying to create a single, static firmware, Open Wrt provides a fully writable filesystem with package management. This frees you from the application selection and configuration provided by the vendor and allows you to customize the device through the use of packages to suit any application. For developer, Open Wrt is the framework to build an application without having to build a complete firmware around it; for users this means the ability for full customization, to use the device in ways never envisioned.
See http://openwrt.org for more info.