The Growling Guitar was my first circuit bending project and also one of my most popular to date. The guitar has two serial-connected potentiometers for fine and coarse pitch control, plus a starve pot in series with the batteries, giving nice, glitchy starve effects.
A toggle-switched capacitor provides a glissando-like effect to note changes. And another switch gives a distortion somewhat resembling fuzz or overdrive.
There is also a toggle switch to chose between internal speaker and jack output.
This instrument is highly entertaining and also very hard to control. It has conswquently failed me on most of our performances, only to come mysteriously to life later on.
A design lesson learned: Never put the jack out connection on the backside of the instrument, as this will make it impossible to use the instrument lying flat on the table, like most circuit bent toys are played.