Celestia is a free space simulation that lets you explore the universe in three dimensions!! Unlike other space simulation software s Celestia doesn't confine you just to the surface of the earth. You can travel throughout the solar system visiting over 100,000 stars!
This is one of the coolest open source software s i have ever come across. The view is so real and fascinating sometimes. Best feature is that it doesn't require a net connection to do all this.
Celestia has support for Windows, Linux and Mac.