A tachometer, or RPM counter, is a device that measures the speed of something that's rotating. In a car with a manual transmission, the tachometer can be helpful in determining when to release the clutch and how much gas to give when you're taking off. This DIY Arduino Tachometer by Chris on PyroElectro uses an infrared transmitter and receiver break-beam pair to measure the RPM of a computer fan.
When the fan blade passes between the transmitter and receiver, the IR beam counts the interruption. The fan has seven blades, so seven interruptions equal one rotation.
You can use a break-beam system for other things as well, like knowing when a door is opened or closed. If you want to build your own tachometer, you can find step-by-step instructions and schematics on Chris's post. Check out the video to see how it works.
I wonder what else this could be used for... maybe it could be attached to a bike? Do you have any other ideas?