My First Arduino Synth
Notes and Volts have some great videos over on Youtube detailing the build of a little Arduino Midi Synth. Originally it lacked MIDI but they've kept adding features with the newer videos and now it's become a pretty useful instrument, as well as a fantastic lesson in Arduino Synthesis;
Part 3 - Line Level Upgrade
Part 4 - Adding MIDI
This was also my first synth that I connected to the mains power supply and quite amazing I didn't electrocute myself!
The project box / enclosure for this synth came from Conrad for €4.50;