Celestia... Space simulation that lets you explore our universe in three dimensions.
Celestia runs on Windows, Linux, and Mac OS X.
http://www.shatters.net/celestia/
Downloads for each OS:
http://www.shatters.net/celestia/download.html
Addons:
http://www.celestiamotherlode.net/
A (not so) Brief Introduction to Celestia Addons:
http://www.lns.cornell.edu/~seb/celestia/addon-intro.html
Download it direct from NASA:
http://worldwind.arc.nasa.gov/