I have ported FreeRTOS as libraries for AVR and ARM Arduinos as FreeRTOSBeta20121215.zip http://code.google.com/p/beta-lib/downloads/list.
I am currently modifying the 16 examples from the FreeRTOS book http://shop.freertos.org/RTOS_primer_books_and_manual_s/1819.htm to run as Arduino sketches. I will be posting these soon.
I have also ported ChibiOS/RT as libraries for AVR and ARM Arduinos as ChibiOSBeta20121217.zip http://code.google.com/p/beta-lib/downloads/list.
ChibiOS/RT ChibiOS free embedded RTOS - ChibiOS Homepage is not as popular as FreeRTOS but is technically very interesting. It is high quality and I use it a lot.