Google Maps Labsadded a feature that used to be available as a mapplet: distance measurement tool. After enabling the Labs feature, you'll notice a small ruler at the bottom of the map. Click on the ruler and you'll be able to use the distance measurement tool by clicking on the map and tracing the path you want to measure.
If you click on "I'm feeling geeky", you'll be able to pick from a long list of measurement
units, including light-year, parsec, PostScript points, Olympic swimming pools, American
footbal fields, Persian cubits and more.