SpaceTheremin is a virtual mouse-controlled theremin. Unlike a standard analog theremin, which typically has a sinewave oscillator and some basic vibrato, SpaceTheremin lets you choose from six different oscillator types (sine, square, triangle, sinc, saw, and pulse).
It has an adjustable pitch range and check-boxes that let you select whether the pitch and volume ranges are logarithmic or linear. The use of the vibrato can result in some incredibly freaky and bizarre sounds.