Celestia - 3D Space Simulator

Celestia

"Celestia is a free, interactive (real-time), 3D astronomy program. It doesn't just show you the sky as it can be seen from earth as most planetarium software does, but allows you to move to and view the universe from any point between the planets and the stars."

Celestia Motherloade - Addon Repository for Celestia

"The Celestia Motherlode hosts over 10 GB of Celestia addons by various creators, which extend or change the way Celestia renders the universe."