A fellow named Julian is making a virtual analogue/FM drum machine with built-in sequencer. The details are over at sonic-potions.de. It looks quite exciting, all open hardware and software. Kits will be available, although the mainboards containing the main microcontroller and audio codecs will come preassembled as they’re smallish SMD components.
Incidentally, it’s based on the same STM4F chip that I just got a development board for. Mine’s yet to make a sound, though, but I’m looking forward to seeing the code and designs for this. And probably snagging a kit!