The Virtual Theremin
I did a little googling and found that it's relatively simple to make one, but I didn't want to, I wanted one now! Back to google it was then, and I came across this beauty (link to .exe file).
To play the Theremin, you click start, then move your mouse around the grid. The X axis controls note frequency and the Y axis controls volume.
I'm now in the process of annoying my parents with it >:]