AvRtos
Small real time operating system for AVR and ARM microcontrollers
...AvRtos provides message queues and semaphores for task synchronization. It uses a static memory allocation scheme.
AvRtos is nearly the smallest RTOS out there. Even pico]OS is much bigger.
AvRtos can be compiled using gcc compiler. There is a AvrStudio 4 and Eclipse project file to get the example code working very quickly.