Celestia
Celestia is an open source, 3D astronomy program for Windows, Mac OS X, and Linux created by Chris Laurel. The program allows user to simulate objects ranging in scale from the International Space Station to the Milky Way Galaxy in three dimensions using OpenGL. Unlike most planetarium software, the user is free to travel about the Universe. Over 1GB of add-on content is available to supplement the basic program as a result of the small, but productive community of users that support the program.