Theremin and guitar-inspired instrument: two buttons to trigger notes, light sensor on the fingerboard to control the pitch, another light sensor to control the "modulation wheel". The built-in pentatonic scale makes it easier to be "in tune" (but not in rhythm...)
Powered by an Arduino board, it controls a synthesizer (here a microkorg) via MIDI. Back beat by Ableton Live.
More details: http://cyrille.martraire.com/2009/03/musical-instruments-for-musicians-and-non-musicians-put-into-practice/