What operating systems have been ported to Cortex-M3?

There are many open source options:

• QP
• BeRTOS
• ChibiOS
• Contiki OS
• Free RTOS
• Micrium uC/OS-II
• eCos
• NuttX
• Zephyr

Most of the above listed have commercial versions as well. I didn't list the strictly commercial RTOS's that are out there, as I assume you are more interested in open source offerings.

FreeRTOS is a popular choice.

http://www.freertos.org/portstm32iar.html

We have used Rowley's CT (started and dropped very fast) and FreeRtos (recommended) in M3s before.

I have been very interested in ChibiOS but haven't had a chance to try it out. Commercial but reasonable.

Also there is M3 specific OS out there called Cocox, looks neat but something bothered me and didn't give it a detailed go. While I was looking at this, I guess, I felt it was too new to take the plunge, perhaps now it is a different story. (14 months ago).