For a special Memorial Day treat, here's a video of the great Clara Rockmore playing the theremin with the delicate vibrato of an opera singer. Just saw the documentary about the theremin called Theremin: An Electronic Odyssey, which features Clara Rockmore, Bob Moog, Brian Wilson, and Léon Theremin himself. Amazing film about an amazing invention.
The theremin operates with heterodyne oscillators for both the frequency (pitch) and amplitude (volume) antennas, using L/C circuitry and grounded variable capacitance (your hand as the "ground") for changing the frequency and volume - much like "tuning" a radio or the electrical equivalent of a tuning fork. As you can imagine, DIY projects for the theremin are plentiful and varied with solar-powered optical theremins that are light-sensitive like this one by the artist Jan van Nuenen (check out his awesome animations, too!):
And back to my love for the arduino, check out this one with infrared sensors called the Squaremin by Nick Hardeman:
And finally, scans from Electronics Illustrated 1960 about the theremin available online thanks to the wonderful website SynthGear.