The Theremin, invented in 1920 by Russian physicist Léon Theremin, was one of the world's first electronic instruments. It's the only instrument that is played without being touched. The proximity of the hands to the two antennae determines the pitch and volume of the sounds produced.
You've likely heard the eerie Theremin before - they became popular in those 1950's science fiction movie soundtracks that sound so cheesy today.
While Theremins are always a popular attraction, they remain pretty rare (I've only once seen a Theremin -- at a shi-shi company event during the dot com boom). A new Theremin costs about $450, so I'm not likely to get one as a stocking stuffer, but luckily thanks to the Wii and the inspiration of Wii hacking pioneer Johnny Lee I've been able to create my own.
To be precise, I've built a Theremin simulator using a computer, a Roland JV-1080 synthesizer, and a Wiimote (remote controller from the Wii game console).
At just $35, the Wiimote is an AMAZING piece of technology. It has an infrared camera in it which tracks the position up to 4 infrared light sources. So I bought a pair of leather gloves, wired up a couple infrared LEDs to 1.5 volt batteries, and poked an LED through the tip of the index finger of each glove.
Then, I connected my Wiimote to my computer (the Wiimote also supports Bluetooth connections): building on top of Brian Peek's Wiimote hacking software library, I wrote a program which detects the two infrared gloves and converts the vertical position of the left hand to volume, and converts the horizontal position of the right hand to pitch. That information is then sent via MIDI to the synthesizer which creates the actual sound.
One awesome thing about this design is that I'm not restricted to the sine wave sound of a traditional Theremin so the sonic possibilities are endless. My father-in-law, a Vancouver Washington dentist, suggested I should make it sound like a dental drill :)
I've had a lot of fun creating my Theremin, and I've learned a lot. I think my biggest lesson, though, is that while playing the Theremin is simple in concept, it's VERY difficult to play well.