I have ported FreeRTOS 7.4.2 to AVR, Due, and Teensy 3.0 Arduinos. The download file is FreeRTOS20130714.zip http://code.google.com/p/rtoslibs/downloads/list
FreeRTOS is the most popular free open source RTOS according to a recent EE Times survey. FreeRTOS is supported on 34 architectures with more than 100,000 downloads per year.
Here are key advantages of FreeRTOS:
Large and growing user base and community.
Free RTOS kernel - preemptive, cooperative and hybrid configuration options.
Designed to be small, simple and easy to use.
Queues, binary semaphores, counting semaphores, recursive semaphores and mutexes.
Mutexes with priority inheritance.
Tutorial books and training to educate engineers.
See the FreeRTOS site for more information http://www.freertos.org/