Mark3 RTOS
Open-source RTOS and Embedded Systems Toolkit for C++ and C
...It currently supports a variety of Atmel AVR, MSP430, and the ARM Cortex-M series of microcontrollers, with other platforms to follow.
Mark3 was designed from the ground-up using disciplined object-oriented design principles - and unlike the vast majority of free or commercial RTOS' is written in C++, with bindings for C-language compatibility.
The carefully designed object model drastically reduces the overall size and complexity of the source code, leading to enhanced code reuse, stability, and modularity. It also makes the source easier to understand - any experienced C/C++ developer should be able to understand the relationship between objects in the system, as well as use and extend them to fit a given application.