Celestia

If you are fascinated by stars, planets and space pics, you have to check out Celestia. For a preview of what it does, go to the screenshot gallery.

Just download te program, run it and press the key 'D' to enter demo mode, then you will be graced by a very nice demo of how the program navigates through space. You can get a lot (and I mean A LOT) of addons for Celestia in the Celestia Motherlode site.

Another amazing piece of software for free.