I have ported ChibiOS/RT and FreeRTOS to Teensy 3.0 as libraries. This allows mixing Teensy software with RTOS functions.
I am now working on ports for Due. It looks like Due will also be able to support these RTOSs as Arduino libraries.
I am also looking at other possible RTOS candidates. Here are my criteria for candidates.
Supports Cortex M3/M4
Free version available
OS Website has forum with activity
Here is my current list of candidates. Some of these would not be easy to port to Due.
First, are there other good candidates that satisfy the above criteria?
Second, what features would other RTOSs have that are not covered by ChibiOS/RT and FreeRTOS?
Clearly QP is different since it is state-machine based. In the past I was a fan of state-machines in embedded systems but I am less enthusiastic now.
It's interesting that after making the above list, I found this list https://sites.google.com/site/stm32discovery/stm32-resources-and-links/open-source-real-time-operating-systems-for-the-stm32-and-cortex-m3-mpus
. It has many of the same systems.