Techdweeb has a great collection of circuit bent gadgets… I really like the “Super Ear Blaster”. The bender writes-

This was one of those generic chinese drum machine thingees, the kind that shares its guts with 1000s of other toys, but the external plastics change depending on who does the packaging. I really like this version of the drum toy because it has pretty lights and sounds great when bent. Very Aphex Twin. It also has dog and frog sounds… This one has two optical resistors for optical sequencing and pitch control that are switchable “on” and has pot control for making it go realllyyy slowwww… I also did a metallic blue with silver splatter paintjob which turned out kinda cool.

More:

Circuit Bending. Modify a Casio keyboard (or other electronic audio stuff) and start playing some of the strangest sounds you’ve ever heard. MAKE volume 04 – page 88.