Surprised nobody has mentioned the MIDIbox SID project with all this talk of SID chips. I know Arduino is more in vogue these days but is there any specific reason people don't like this project? I assume most of you must know about it... I believe MIDIbox/UCapps was the first open source DIY MIDI controller project  I knew about personally anyway :)

http://www.ucapps.de/midibox_sid.html