Image Scanning Sequencer -- has a strip of photo cells that can trigger MIDI notes based on what they're picking up. The darkness of the detected "note" sets the velocity of the note, and the location sets the pitch. The Arduino sits in between, making all the magic happen. If this all sounds completely meaningless and pointless, you're probably right, but check out the video after a break for chance at making sense of it all. And simulated whale noises.
[Via MAKE, thanks Manny]