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.