I've been looking into how to build a custom MIDI instrument using the Arduino that I can connect to my laptop running Pure Data. The following are some good resources on this matter.
MIDI
Wikipedia's MIDI page (yes, of course)
MIDI Communication article on tigoe.net
MIDI Protocol Guide by Hinton Instruments
Arduino+MIDI Hardware
MIDI Output Using an Arduino (from ITP Physical Computing)
Controlling Garage Band with Arduino (from ITP Physical Computing)
Arduino Note Player (from arduino.cc)
There are some nice MIDI shields available on SparkFun that include just the essential components needed to build a MIDI instrument with a MIDI output port.
Arduino+MIDI Software
Arduino MIDI Library
Arduino Shields
MidiVox shield for Arduino
Arduino Piano Squealer Synth (and on Make as the Pocket Piano)
Pd
Using MIDI (FLOSS Manuals)
Configuring Pure Data (FLOSS Manuals)
Programming Electronic Music in Pd (Johannes Kreidler)
Goodbye.
