The 'Color a Sound' converts a overhead projector into a playful color-coded instrument.

Over at Make blog:

A demo video of my installation that uses an old overhead projector, a camera, some markers and a laptop and turns it all into a playful art piece. It is essentially an inverse color organ that you can play like a player piano. You can draw crazy things for fun or make more complex songs if they look more MIDI-sequencer like. Developed in Max/MSP and Jitter.

[via Make]