DuinOS
http://robotgroup.com.ar/duinos/wiki/index.php?title=Main_Page (http://http://robotgroup.com.ar/duinos/wiki/index.php?title=Main_Page)
DuinOS is a small operating system with preemptive multitasking for Arduino and AVR based boards. It's based on the robust FreeRTOS kernel.
Getting Started (http://http://robotgroup.com.ar/duinos/wiki/index.php?title=DownLoad_and_Getting_Started)
Multitasking On An AVR (http://http://www.avrfreaks.net/modules/FreaksArticles/files/14/Multitasking%20on%20an%20AVR.pdf)
Integrating Microchip Libraries with a Real-Time Operating System (AN1264) (http://http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en544728)