DeanMiller has created his own awesome guitar synth based on Arduino. It works similar to a regular guitar, in that you hold a note with your left hand on a 'string' and play the string with your right hand. This clever concept uses a ribbon potentiometer for each string, and a rubber button pad for each string to activate the note. This makes it similar to playing a guitar, so after some practice, he can certainly rock out on it!
The neck of the guitar which holds the ribbons is actually from a real guitar - a Squire Affinity Stratocaster. the frets have been removed to allow the ribbons to sit nicely, and allow the 'bending' of notes by sliding up and down the ribbon. The main body was created himself by routing out holes for the components in his own DIY CNC machine.
A joystick has also been added to to control various effects over MIDI. It acts as an XY controller, allowing 2 different effects to be controlled at once. An Arduino Micro is used to transmit the notes over serial as MIDI data, and a simple MIDI-to-USB converter is used to interface it with a computer. This allows it to plug into his music software, of even hardware synths!