Piezo elements come in handy when you need to detect vibration or a knock. You can use these for tap or knock sensors pretty easily by reading the voltage on the output. They can also be used for a very small audio transducer such as a buzzer.
**Documents:**
* [Datasheet](http://www.sparkfun.com/datasheets/Sensors/Flex/p37e.pdf) (7BB-20-6L0)
* [Arduino Tutorial](http://www.arduino.cc/en/Tutorial/Knock)
**Videos**