[Jacob] is working on his final project for the Copenhagen Institute of Interaction Design. Based around Arduino, the quality and quantity of his build notes make this a fascinating read and there are several examples to listen to.
The project features a brilliant idea for input: He uses a 1/8″ TRS connector (mini-jack) whose tip is the input to the DAC of the Arduino. There are conductive pads in the shape of a keyboard that you touch the tip of the connector to in order to complete the circuit. Alternatively, the other two conductors on the connector deliver power and ground for easy interface with external controllers. He built an example controller that uses an LED and photoresistor to alter the signal returning to the Arduino. Put your hand in front of the light and the sound changes.
[via Arduino: blog]