NFC Plugs have released Particle Fountain, an unusual free VSTi synth for Windows.
Particle Fountain is a monophonic synth that attaches a sound (either a sawtooth or a square wave) to a bunch of particles, which move in a 2d space like a fountain. The particles' position in the space then determines certain aspects of the sound (frequency, amplitude, panning). There can be up to 200 particles in use at any one time.
They have also made a special limited edition Christmas version with white particles and a snowman!