A long time I ago I put up the Beakman's Motor Page (http://fly.hiwaay.net/~palmer/motor.html). When I got my Arduino Diecimila, I decided to make an optical tachometer to measure the speed of these simple motors. I did this to both teach myself a little about Arduino development and because I get asked repeatedly how to measure the speed. This may not be the simplest way, but at least it works and I have something to tell someone when they ask.
The project is up on Instructables: http://www.instructables.com/id/EDTP9R3F6B7T1Q9/
More pictures at: http://flickr.com/photos/cmpalmer/sets/72157602039875912/