The Original Pixelh8 Music Tech Game Boy Synthesizer is software that runs on an unmodified Game Boy or Game Boy Advance, enabling you to be able to control the game boys on board sound. You can edit the wave duty, pitch sweep and volume envelope as well as new additions such as unison, 5th power chords and octaves, turning the Game Boy into a real time instrument.

http://pixelh8.co.uk/download/