Celestia is definitely one of the best open-source software that I have ever tried. I followed the link to this software via Dave. Celestia is a 3D-based simulation of outer space. You can explore the solar system, go to the nearest stars, and see in three dimensions the arrangement of the Local Group of Galaxies (which includes the Milky Way, the Large and Small Magellanic Clouds, and the Andromeda Galaxy).

I wish I had a really fast processor and a really good 3D-accelerator video card so that I can fully appreciate this software. I could waste hours on this! Highly recommended.