The free space simulation that lets you explore our universe in three dimensions. Celestia runs on Windows, Linux, and Mac OS X. Unlike most planetarium software, Celestia doesn’t confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy. [via] Link. Bonus link- lots of add-ons.
Celestia – Free planetarium software