Chladni Plates are flat vibrating metal plates which, when sprinkled with salt, show very interesting patterns. If the salt is replaced with a liquid, you can get 3D patterns.

You can see this for yourself if you take a polystyrene cup of black coffee and move it slowly across a flat surface (you may need to apply slight pressure). The cup vibrates, and the vibrations are passed onto the liquid, appearing at the surface as a series of quite pretty standing waves.

I've also seen similar patterns at the event Seven Eighths Of A Second, which used real sounds rather than standardised tones to drive the process.

• Good selection of images - http://groups.physics.umn.edu/demo/waves/plates.html
• Links to real pictures - http://www.physics.utoronto.ca/nonlinear/chladni.html
• Interesting illustrations and some information on pattern recognition using harmonic resonance - http://cns-alumni.bu.edu/pub/slehar/webstuff/hr/hr.html
• Nice pictures of the equipment - http://physics.kenyon.edu/EarlyApparatus/Acoustics/Chladni/Chladni.html