Chladni Plate

Chladni Song by Meara O'Reilly, film by Lisa Foti-Straus

This phenomenon was known at least as far back as 1680. Ernst Chladni published his technique to demonstrate the phenomenon in 1787 (wikipedia: Ernst Chladni), using a bow against a piece of metal sprinkled with flour. Nowadays a speaker is placed under the plate to more accurately control the amplitude and pitch of the signal to be displayed. Which actual patterns appear are affected not only by the pitches and resonance, but also by how the plate is physically attached and (presumably) the location of the speaker.

Here's a good explanation: Chladni Plates

And here's how to build yer own, via Make magazine: http://www.make-digital.com/make/vol16/?pg=128