Celestia
Celestia is an open-source, photo-realistic, real-time, three-dimensional viewing of the solar system, the galaxy and the universe. Celestia is an easy to use, freely-distributed, multi-platform, open source, software package which has become a valuable tool for astronomy education. Used in homes, schools, museums and planetariums around the world, it also is used as a visualization tool by space mission designers. Versions are available for computers running Windows, Macintosh (Mac OS X) and Linux operating systems.
Although it is optimized for 3D astronomical visualization, Celestia can be used to display and explore other 3D environments, too. Celestia is an open-source project. As such, its source code is provided and is freely modifiable and redistributable as per the GNU Public License. To download, click on http://www.shatters.net/celestia/download.html
Site: http://shatters.net/celestia/
Forum: http://shatters.net/forum
Wikipedia: http://en.wikibooks.org/wiki/Celestia