There is no way in which a country can satisfy the craving for absolute security, but it can bankrupt itself morally and economically in attempting to reach that illusory goal through arms alone.
 
Dwight D. Eisenhower
 
The guns and the bombs, the rockets and the warships, are all symbols of human failure.
 
Lyndon B Johnson

Our World

Celestia

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

WorldWind

Download it direct from NASA:
http://worldwind.arc.nasa.gov/