In addition to documenting the build steps to create something called a Rubens' Tube, I'll also go over some of the basic concepts and science involved with sound waves.
From the moment I discovered what a Ruben's Tube was, I wanted to build one... and considering I was in a physics class at the time, there couldn't have been a better excuse than extra credit.
Update: I'm thrilled to see that this Instructable's become popular, and I'm more than happy to have people build on the project, but please respect the Creative Commons BY-NC-SA license.
Step 1: What is a Rubens' Tube?
This original Rubens' Tube was a four meter section of pipe with 200 holes spaced evenly along its length. When the the ends are sealed and a flammable gas is pumped into the device the building pressure will have only one route to equalize. The escaping gas can be lit to form a row of roughly even flames. Upon introduction of a loud speaker to one of the sealed ends, standing waveforms can be seen in the flames.
Within the Rubens' tube, as soon as gas is ignited generally uniform flames will be seen. This is because there is very little pressure differential between any given area of the space inside the tube. Once sound is applied from one end, pressure will change within the tube. Should the sound be an easily measurable frequency, the wavelength will be visible in the series of flames, with the highest flames being where condensation is occurring and the lowest where rarefaction is occurring.
They can also be used as an incredible visualizer for music.
(Note that I've muted that audio intentionally, as I don't own the copyrights to the music being used in the clip, and I'm not sure what the Instructables policy on "fair use" is. Searching on Youtube or Metacafe for "Rubens Tube" will show you several videos with music.)