If you do any web development you owe it to yourself to download the Firebug extension for Firefox. Check out the website for details. I just downloaded it and have already used it to trace all the inherited CSS properties for a particular HTML element, saving me lots of troubleshooting time.
I’m a big fan of Google Earth and not just because it’s cool in itself but because of all the resources that extend it’s functionality. The Google Earth blog contains a wealth of resources for getting more out of Google Earth. For example I recently came across this entry that describes how to get an animation showing the progress of continental drift over the last 650 million years or so. Other recent posts include a map of glaciers showing which ones are advancing and which ones are melting and a map of the flood in Florence in 1966 (important for Art History students!).