"µOS++" (micro OS plus plus) is an open source, royalty-free, real-time operating system intended for embedded systems built around ARM Cortex M or AVR devices. It is based on a preemptive scheduler written in C++ (more info in the Wiki).
ButterflyOS is a very simple operation system for the Atmel AVR Butterfly board. It's based on the AvrX multitasking scheduler, created by Larry Barello. A byte-code program for ButterflyOS, can be sent and managed through the serial interface.