If you've ever been curious about the speed of a fan or other rotary device then you will find the following project by Instructables member electro18
of interest. They've devised a neat and non-contact method of measuring RPM that only needs a small white piece on a section of the rotating device.
The tachometer then uses infra-red LED and a photodiode to bound IR light off the rotating device, and detects when the white piece has passed - and thus a revolution. Wiht the use of interrupts in the sketch the tachometer can measures speeds of up to around 20000 RPM. A quick demonstraiton is shown in the following video:
This could also be the basis for other projects such as making a speedometer for a bike or measuring the RPM of rotating shafts, pumps and so on. Either way, visit the project Instructable page for complete details.