A downloadable desktop theremin
Free 30 day trial of 'Mousing' from Sagebrush. This program
has the advantage of being highly configurable. You can output your
mouse movements via your soundcard's wave output for that traditional
theremin sound, or you can 'play' over MIDI, making this program a fun
alternative controller for your keyboard. Some quite astonishing
effects are available.
Visit Sagebrush's homepage for more information at http://www.sagebrush.com/