I was reminded of this while reading (or rather; watching) up on trigonometry (check khanacademy.org, it's excellent!).
You can create any waveform simply by superimposing sine waves.
So here's a simple schematic that lets you do exactly that and nothing much more.
There's 20 sliders, each one controling a harmonic frequency of the base frequency played through midi.
There's nothing revolutionary or particularly useful in here, but it's fun nonetheless.
p.s. You can modify it to include "fractional" harmonics to make the amplitude oscilate, but since these aren't what I was after, I left them out.

















