ARM Cortex-M3 development tools?
I have been playing with an STM32 (also Cortex M3) lately in my spare time and using the CodeSourcery distribution of GCC, which has worked out pretty well.

A colleague who has worked with ARM micros professionally in the past told me that he was satisfied with the IAR toolchain, although I don't know what the cost is like or how the Cortex support is.