My SO recently found Gmaps Pedometer, an applet that allows you to precisely measure the distance you walk (or bike, or drive) using Google Maps. It's easy to use (just find your starting position, click "start recording", and then double-click to add waypoints), and allows you to collect some pretty neat data. Especially cool is the ability to plot your own path; for example, if you cut through a park on your way to the store, you can map that out and calculate the distance.
I now know how far I have to walk to check my mail on campus (too far), and exactly how far it is to the grocery store (nice and close!).
(Oddly enough, Jill just linked to this as well, though I only read her post after writing this one.)