Google Earth (their homepage) is one of only two things on my top 12 list that requires a download and install. Still it is free and relatively light-weight. There are also paid premium versions of GE, but the free version is just fine for normal use.
The Google Earth Blog is a useful resource. Here is a page with links to info about the basics, such as how to move your view with zoom, pan, tilt and what the different settings will do.
Just to demonstrate how it can work, I’ll make a screencast of Google Earth showing a few of the destinations in and around Duluth, MN. This will be practice for a better one that I will post later on Desire2Blog for the D2L User Conference in July 2007.
You should have an Internet connection when using Google Earth as the download itself does not give you full functionality. It is possible to use it for offline viewing (no Internet connection), but it is somewhat limited.
You can even follow Santa’s trip around the globe on Christmas Eve. There are games you can play with GE such as Google Earth War, Google Earth Game, and GE Chess.
Check out the Google Earth Education Community. For a whole bunch of other links and more info, check out the Wikipedia page about Google Earth.