I know very little about theremins. I know that you play them without your hands touching them, that the Beach Boys used one to make the weird noises on "Good Vibrations", and that a bunch of old sci-fi movies used them for eery sounds. Other than that, squat. I'm ignorant of all of the technical mumbo jumbo. There's a couple links below to Wikipedia entries if you're that into it. But the rest of you knuckleheads might find the online theremin simulating doohickey amusing. I just messed with it for a couple minutes and was able to replicate, accidentally, some familiar sounds. One hint, keep your right clicker down while you move the cursor around willy nilly. Depending on you propensity to indulge in time sucking distractions, you'll either find it very cool and fuck with it for hours on end, or, as I did, shrug meat. Cool nonetheless.
Theremin at Femur Design Online version, obviously not the real thing.
Theremin at Wikipedia
Léon Theremin at Wikipedia