Sparkfun: Large Piezo Vibration Sensor - With Mass
The Large Piezo Vibration Sensor is a basic piezo sensor which is often used for flex, touch, vibration and shock measurements. This sensor acts like a small capacitor which discharges up to +/-90V when the film moves back and forth. Some common uses this type of sensor are for impact sensing or as a flexible switch. A larger scale example are piezo sensors that are laid within the road for use of traffic signals.
This sensor can be purchased at SparkFun.