FunkOS

FunkOS is an RTOS for 8-32 bit MCUs including AVR, MSP430, and Cortex M3. Both C and C++ kernels avaiable. Key features include preemption, periodic events, device drivers, and mutex/semaphores. Includes a variety of graphics and filesystem middleware.

Please also check out FunkOS's big brother, Mark3OS. Mark3OS is slightly larger, but is more scalable, much more feature-rich, and supports a wider variety of devices.