freeRTOS for Arduino Uno, Goldilocks 1284p & Mega 2560
Low power application-configurable Operating System
Open-source RTOS and Embedded Systems Toolkit for C++ and C
PRTOS is a simple, compact preemptive RTOS for embedded systems
Mulithreading Arduno library
Small real time operating system for AVR and ARM microcontrollers
Simple AVR OS