I have posted version 7.3.0 of FreeRTOS as FreeRTOSBeta20121215.zip http://code.google.com/p/beta-lib/downloads/list
. A number of examples are included.
FreeRTOS is very popular and good documentation is available http://www.freertos.org/
FreeRTOS supports cooperative scheduling but I like ChibiOS/RT a little better in cooperative mode so I have not provided an example of this mode. ChibiOS/RT is also posted as ChibiOSBeta20121212.zip and has a coop example.
A small patch is required to run FreeRTOS on Due. This patch involves a hook for access to the Cortex M SysTick interrupt. Future versions of the Arduino IDE should have this hook. I included the patched version of wiring.c for 1.5.1r2.
I have packaged FreeRTOS as two libraries, FreeRTOS_AVR and FreeRTOS_ARM. It would have been possible to combine these into one library but that would require a lot of modification to combine of the original FreeRTOS files with conditional compilation.