From the MAKE Flickr pool

Member Recotana may have built the smallest MIDI input device ever. It uses a PIC12F675 chip, a few basic components, and single switch to register input. He plans to use it in conjunction with a heartbeat sensor – cool! This could come in handy for quite a few project ideas. –Link

Homemade MIDI Controller
Homemade MIDI controller –Link

From the Maker Store:
MidiTron, unassembled kit –Link