4. The Theremin
Okay, if you've never heard of a theremin before, you need to drop what you're doing and watch it in action. This bizarre electronic instrument uses antenna to sense where the player's hands are, and intone certain notes accordingly. Not only do thereminists appear to be plucking invisible strings, the sound itself is ghostly and ethereal. With its appeal to both electronics buffs and fans of the occult, the theremin is a true geek twofer.