Bitbeam is a 3d printable, LEGO Technic-compatible construction system. More info at http://bitbeam.org

Use this shaft encoder wheel to measure the rotation of a wheel. (You'll also need an LED and a phototransistor.)

Shaft encoders are especially useful when using a DC motor for motion. DC motors are cheap and wonderful for making little robots move, but DC motors do not come with a built-in feedback mechanism for precision control. Shaft encoders are a great way to get that feedback. For an explanation on how to use shaft encoders in your projects, I recommend Fred Martin's book "Robotic Explorations".