Even if you don't know what circuit bending is, you've definitely heard what it does. Although it's front and center in glitch, Avant-garde and noise music, elements of it can be heard in main stream genres too.
So what is circuit bending? The art of creatively rewiring, customizing, and shorting an electronic circuit. Usually it's tiny, low voltage electronics - guitar effects, children's toys etc. It's a really neat process, and something that tickles the experimenter in me!
Reed Ghazala is the acknowledged "father" of this practice. He might not be the first one to do it, but he's definitely the one to coin the term and spread it as a musical instrument. Here's a short documentary that's both informative and fascinating. Makes me want to go out and dig up all my half-dead electronics!