I have posted a new version of FreeRTOS as FreeRTOS20130521.zip http://code.google.com/p/rtoslibs/downloads/list.
This version is compatible with the new malloc in Arduino 1.05.
FreeRTOS is a very popular RTOS from Real Time Engineers Ltd.
The FreeRTOS_AVR library supports AVR Arduinos. The FreeRTOS_ARM library supports the Due Arduino, and Teensy 3.0.
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.