Celestia is a free real-time space simulation that lets you experience our universe in three dimensions. Navigate through space and visit Saturn, Mars and the Sun!

http://www.shatters.net/celestia/

Direct download link: http://prdownloads.sourceforge.net/celestia/celestia-win32-1.5.1.exe?download

The graphics are of very high quality. It might take some time to learn how to use all keyboard controls required to navigate in real time through our solar system and outer space, but it is definitely worth it!

Celestia runs on Windows, Linux and Mac OS X.