Recently I discovered a website which supplied free and open development tools specially for ARM Cortex M3, it looks good，now share with you:
http://www.coocox.com

CoOS Features:
Free and open real-time Operating System
Specially designed for Cortex-M3
Accords with CMSIS
Supports preemptive priority and round-robin
Scalable
Interrupt latency is 0
Can avoid priority inversion
Stack overflow detection option
Semaphore, Mutex, Flag, Mailbox and censored for communication & synchronisation
Supports the platforms of ICCARM, ARMCC, GCC

I just visit http://www.coocox.org for this OS
It seems that they have made a update: CoOS v 1.1.4
CoOS Features:
Free and open real-time Operating System
Specially designed for Cortex-M series
Scalable, minimum system kernel is only 974Bytes
Adaptive Task Scheduling Algorithm.
Supports preemptive priority and round-robin
Interrupt latency is 0
Stack overflow detection option
Semaphore, Mutex, Flag, Mailbox and censored for communication & synchronisation
Supports the platforms of ICCARM, ARMCC, GCC
Here is the release note:
http://www.coocox.org/CoOSGuide/CoOSUpdates.htm