Celestia is a free opensource space simulation software that lets you explore our universe in three dimensions. Celestia provides photo-realistic, real-time, three-dimensional viewing of the solar system, the galaxy and the universe and has become a valuable tool for education in Astronomy. Celestia is used in homes, schools, museums and planetariums around the world, it also is used as a visualization tool by space mission designers.
Celestia runs on Windows, Linux, and Mac OS X. The latest stable version is 1.5.1.