ChibiOS/RT is a compact and fast RTOS designed for embedded applications. It offers threads, mutexes, semaphores, messages, events, timers, flexible I/O with timeout capability. ARM7, Cortex-M3, AVR, MSP430 and x86 are currently supported.
scmRTOS is a free tiny preemptive Real-Time Operating System intended for use with Single-Chip Microcontrollers. It has very small footprint (from ~1K of NOR Flash for code and 512 bytes of RAM for data). Supported platforms: MSP430, AVR, STM8, Blackfin, ARM7, Cortex-M3, Cortex-M4. This RTOS is written in C++ language.