The Makey Makey is a $50 invention kit that lets you turn just about anything into a controller, for music, gaming or whatever else you imagine.
In a nutshell, the Makey Makey works as an interface between alligator clips, that you can clip on physical objects, and your computer. Attach the alligator clips to things and then start using them as controllers.
Here’s the official intro video:
The kit includes the MaKey MaKey, USB Cable, 7 Alligator Clips, 6 Connector Wires and Quick Start Guide. See the Makey Makey site for details.