The A_SID emulates a Commodore 64 sound chip (the famous SID) using an ATMEGA microcontroller. Vblank did a workshop on the A_SID during this year's Blip Festival.
I've made a simple MIDI --> Serial --> A_SID setup. It works like this:
1. Max/MSP Patch - route your MIDI data to this patch
2. Arduino Code - download and then upload to your Arduino board
3. Arduino Hardware - connect 5V, GND and TX to the correct points on the A_SID circuit (essentially replacing the existing serial connection header if there is one).
Get these materials here. Still working to smooth out some of the details, so no point in really using it now. Quick demo of Ableton Live control below.