Electro18 posted a tutorial on how to make a portable digital optical tachometer using an Arduino Uno, an instructable here:
A tachometer is a device used to measure the RPM or Revolutions Per Minute of any rotating body. Tachometers can be contact based or non-contact ones. The non-contact or contact-less optical tachometers usually use laser or Infrared beam to monitor the rotation of any body. This is done by calculating time taken for one rotation.
It can measure RPM over 20k
Sensor range extends upto 7~8 cm
Displays Maximum RPM
Measure RPM – DIY portable digital tachometer – [Link]