Theremins make some spookiest sounds around, but they are notoriously difficult to play. This homemade version uses IR light instead of induction loops, and if you can slide a card across a table, you can play it.
Built by Chris at Pyro Electro, the Mini IR Theremin uses IR sensors to detect the distance between the Theremin and a light reflecting obstacle like a card or your hand, and translates this into a musical note at a certain pitch. By moving your hand to different distances, you can vary the pitch up and down to play a tune.
Unlike with a real Theremin, there is no volume control, and the pitch changes are in distinct steps rather than the original's smooth glissando. The result is something that sounds kind of like a cheap synthesizer, without all of the spookiness that makes the original so cool.
Still, it looks pretty easy to build, and even easier to play.