Celestia is an awesome planetarium application letting you “experience our universe in three dimensions.” Its massive database lets you explore the Solar System and beyond; try the included demo (under “help”) for starters and then play to your heart’s content. What’s even better is that not only is the software freeware but it is also allows add-ons for new objects, textures, and more: this site and this site are good starts, although there are more out there. I was really blown away by Celestia; download it and see for yourself!
