If you are taking the DIY approach to coding for the ARM processors, you probably use the GCC ARM embedded toolset. This is available from a number of places. I just came across another distribution of this ARM toolset. One immediate benefit is that they provide pre-compiled binaries for Windows, Linux and the Mac.
Edo Franzi is the designer of the famous Khepera Robot. He first developed uKOS, a multi-tasking OS for the 68k processors, as part of that project. uKOS was subsequently re-written in C and is now available, as uKOS-II, for a range of processors including the ARM Cortex M3 and M4 processors like the STM32 series.