ST Micro are selling very cheap ARM Cortex-M development boards.

As an introduction to their Cortex-M0 family, the STM32F0 Discovery can’t be beat. I got a couple free at a trade show, but since they are only USD8.00 you should pick up a couple if you’re interested in the M0.

STM32F0 Discovery board

The chip on the board is an STM32F051R8T6. Briefly, its specs are: