Celestia1.5.1

Celestia is a real-time space simulation that lets you experience our universe in three dimensions.

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. All travel in Celestia is seamless; the exponential zoom feature lets you explore space across a huge range of scales, from galaxy clusters down to spacecraft only a few meters across. A 'point-and-goto' interface makes it simple to navigate through the universe to the object you want to visit.

Although it's not absolutely required, the program will be vastly more enjoyable if you also have a 3D graphics accelerator in your computer.