This video, via Robot Room, demonstrates how you can make a mechanical waveform generator from an old turntable, a simple light sensor circuit and some laser-printed disks.
A white LED provides a consistent bright light source that reflects off the printed waveform disc into the light sensor. When a dark portion of the disc rotates underneath the light sensor, less light is reflected, and thus the voltage is lower on the output.
See the Robot Room site for more details.
Check it out and let us know what you think of it in the comments!