If you've ever seen the Blue Man Group, then you know all about the Tubulum. This instrument is built from a series of PVC tubes that are whacked at the end with flat rubber slappers, similar to cheap flip flops.
We've come across several free plans online for Tubulums such as the eHow version, but nothing compares to the college physics report authored by John Johnson that is swimming around the internet.
Johnson's plans gives precise measurements of the tubes for proper tuning, even noting the temperature and humidity of the area, which was affecting the tuning.
Download the plans and build your own!