MIDI Output using an Arduino - Fantastic tutorial. The best I've seen
Roguescience Arduino Tutorials part 4: add a pot
MIDI Messages
midi-control-change
Control Changes
Arduino MIDI Volume Pedal
Download the Arduino Software
Installing Arduino on Linux
MIDI Library at the Arduino: Playground
Great series from little-scale.blogspot.com:
Arduino MIDI out example
Pots and switches to midi data
More pots and switches to midi data