I've known about this for some time now, but I never realized how amazing it was until I just tried it out now. The WorldWide Telescope from Microsoft Research lets you explore the universe on your PC. I just got done navigating through the entire solar system, checking out all sides of planet Earth, and looking at the view of the moon from Apollo 12. Absolutely incredible!

Still not convinced it's worth installing? Check out the tour. I think you'll be impressed.

 

Here are a few photos I took during my exploration...