Vacuum cannons are devices that accelerate an object, usually a ping pong ball, by drawing a vacuum inside a long tube that has membranes stretched over both ends, then bursting one of the membranes and allowing air to rapidly rush in. These cannons can accelerate a ping pong ball to supersonic velocities, allowing it to penetrate aluminum cans and other targets.
Vacuum cannon are potentially dangerous and must be treated with great caution. At such high velocities even such a light projectile could easily "put your eye out, kid."
This post will collect links to videos, construction information, and calculations and experimental results for vacuum cannon ballistics.
This Idaho State University site has one of the best instruction sets, emphasizing the vital membrane sealing and retaining factors. A bad membrane can only produce a bad cannon. The extremely rapid bursting of the atmosphere admission membrane is critical to the rapid acceleration of the ping pong ball. Let in the same amount of gas slowly and much of the air will simply flow around the ball, rather than pushing against it.
This parent directory at Idaho State contains many links to physics demonstrations. All are free.
This is an "instructable" that gives complete directions for a build-your-own vacuum cannon.
This youtube video shows a physics instructor demonstrating and explaining his vacuum cannon.
This "improved" vacuum cannon has some nice innovations. A well-designed can holder should improve pop can penetration and the transparent cannon is a great idea.