Here are my links to my "Out of this world freeware". WARNING: Only download when you have some time to just play. It is very easy to loose a lot of time with any of these.
NASA World Wind: Cool program for visualizing the Earth in 3D. There are a number of cool plugins that people have created to map data in, including: Earthquakes and Volcanoes (shows plate boundaries quite nicely), near real time satellite photos, historic earth surface temperatures, ocean currents. The newest version includes the Moon, Mars, and the Sloan Digital Sky Survey. (Windows only)
Celestia: Tour the universe in an interactive 3D environment. This is really cool and people have created lots of add on stuff including educational units. Check out the Celestial MotherLoad. If you're going to use the educational units you'll need to download the version of Celestia from the educational unit page. The educational units are great! They include information in Word Documents with links into Celestia, so you read about a planet/object and then you get to interact directly with it. (Windows, Macintosh, Linux)
Stellarium: Interactive planetarium in your classroom. You tell the program where you are in the world and it generates a 3D picture of the sky. See a star and click on it for some information about that star. You can even zoom in on planets or nebulae to get a close up view. It is possible (I've not done this) to take a series of pictures from your backyard and "skin" Stellarium so the program will look exactly like what you will see when you step out side. Stellarium will draw constellations or show constellation art as well. I haven't used Stellarium in class, but Ben Rimes over at The Tech Savvy Educator has had success using it with his 6th Grade students. (Windows, Macintosh, Linux)