I’ve just released a DIY kit called the Groovesizer. When I wanted to start learning about DIY music electronics I got myself an Arduino Uno board and started to experiment with all of the cool audio projects that have been created for the arduino. I thought it would be useful to make a PCB that collected all the hardware for the different projects on one board, and that one could switch between with jumpers.
Long story short, the fruit of these labors is now up for sale at http://groovesizer.com. I’m in the process of making all the design files and code available, too.
The Alpha firmware features a 32-step sequencer and (basic) 3 voice synth. More will follow soon.