[Andy A.]

A gear tooth sensor can be a good stand in for an optical encoder. By sensing the gear tooth count on a known gear, and knowing the reduction between that gear and what ever objects speed you want to know, you can deduce speed.

They are sometimes called 'Hall effect' sensors. This is more accurate as you can use them to sense any ferrous object, not just gear teeth. Bicycle computers use a hall effect sensor, I believe. Cellphones also use them sense when they are open. There are all sorts of cool applications, although counting teeth is the unglamorous role they usually get in FIRST.

-Andy A.