The Virtual Theremin: "The Theremin is a musical instrument that uses the positions of the performers hands to create sound. The Virtual Theremin aims to mimic the Theremin using just a computer and a cheap web camera. It tracks the hands of a performer, using image processing techniques, around a picture of a Theremin superimposed on the video stream. The positions of the hands are then converted to sound that mimics the response of the Theremin." (Via Linkfilter.)