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). The same waveforms can also be used for vibrato.
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.