Here's something new: a musical instrument which requires no physical contact at all.
The Theremin is actually an old instrument, invented in the 1910s. Wikipedia will tell you all about it. I just stumbled upon (haha) it while reading from gizmodo, where a motor was used to control the instrument.
Here are some great videos of the Theremin
The original Gizmodo
An introduction to the Theremin
Clara Rockmore - One of the best videos I found. Her Wikipedia page gives more information.