Chibios/RT is a freely available RTOS written and maintained by Giovanni Di Sirio.

It is available for many CPU architectures.  Of particular interest to me are the Atmel AVR processors and the STM32 processors.  Chibios can be configured to provide useful functionality even on the AVR processor in the Arduino Uno with 32K of Flash and 2k of RAM.

You can find more information about Chibios at:

As with many projects that are supported on a part time basis, the documentation is a little sparse in places.  In this case, most of the documentation is provided through Doxygen and a large collection of example programs.

This leads to an unfortunate combination of API docs that lack detailed information on usage and parameters, and a set of examples that are intended for advanced users and include a lot of complex functionality that makes the examples hard to follow.

On these pages, I hope to help fill in some of the rough spots getting started with Chibios.