The STMF4 Discovery is another inexpensive ARM Cortex-M development board from ST Micro.

The board sports quite a capable processor, handily capable of doing some DSP. With the built-in mic and headphone amp, this could be a fun cheap board to explore digital audio with. When I last looked they cost about USD14.00.

STM32F4 Discovery board

The chip on the board is an STM32F407VGT6. Briefly, its specs are: