Ernst Chladni invented a way of visualizing the vibrations on a metal plate by sprinkling sand on the plate and making it vibrate using a violin bow. The sand scatters and collects at the places where the plate is not vibrating (the nodes of vibration), creating a fascinating family of visual patterns. The images below are artistic renderings of some of the many possible patterns. See more at the U Of T Physics Department and more gory details on the math from Paul Bourke—including some visualizations of what Chladni patterns in three dimensions would look like.
[Unfortunately, I haven’t yet found a good applet which demonstrates this behaviour. (Sounds like a perfect project for proce55ing!) If anyone knows of a nice visual demonstration, please let me know and I’ll update this entry.]