I love playing around with this web-based Theremin by Femur Design. (There is also a $3 desktop/mobile app version). It's described as a "free online synth instrument that makes an array of different sounds" and It's easy to use — just drag your cursor around or use your fingers with a touch screen.
You can experiment by messing with the waveforms, delay, feedback, and scuzz sliders. I've only had the chance to try an analog theremin out one time, and it was way too much fun. If I had one at home, I don't think I'd be able to walk away from it.
What makes a real theremin special is that it can be controlled without any physical contact. The theremin was patented by Leon Theremin in 1928 and is considered one of the first electronic instruments.
The instrument's controlling section usually consists of two metal antennas that sense the relative position of the thereminist's hands and control oscillators for frequency with one hand and amplitude (volume) with the other. The electric signals from the theremin are amplified and sent to a loudspeaker.
The theremin has a trance-inducing sound that always makes me think about aliens and outer space.