(Copy/paste from Wikipedia)
Circuit bending is the creative short-circuiting of low voltage, battery-powered electronic audio devices such as guitar effects, children’s toys and small synthesizers to create new musical instruments and sound generators.

Thanks to Chesterfied, I could have a live demo of bent instruments. Great!
Below is one of them (more videos on his YouTube account).

If you miss the 8-bit sound of your old beloved NES, then you should like this track by Maru…

Maru: Fairy tale confusion

Maru and USK have created Portalenz, one of the top chip tunes group in Japan. They use gameboys and bent instruments (TI speak & spell for instance) on stage. Rock on!

Some links:
casperelectronics, this guy is crazy about circuit bending, really.
MySpace: Chesterfield, Maru, USK