The Ruben's Tube is a simple project that let's you see sound using fire.

Using a simple length of PVC tubing, a 2 inch audio speaker, and some propane, this guy creates a pretty neat result. The tube is drilled with small holes at even intervals which allow the propane to escape. The propane is fed into one end of the tube while the other end is fitted with a small 2 inch audio speaker. As the speaker is driven with various frequencies, you can see the sound waves depicted in the flames emanating from the tube.

The basic sine wave pattern is shown first with a couple basic frequencies but the real magic happens when music is fed into the speaker. How cool would something like this be in a bar or restaurant?

Oh, and don't try this on your own - you could get hurt.

More details and photos can be found here.