The Coltar (or Colour-Guitar for those of you not as cool!) looks like a fantastic little project.
YouTube user Philipp Lenssen has put together a nifty little guitar that gets its cues from a coloured fingerboard and a colour sensor. Place the sensor over a different colour and get a different note - simple but effective.
What makes it stand out over similar projects (like my very own sequencer) is that a button on the side changes the chord that the colours relate to. This allows him to create songs by following a set chord progression.