The Microsoft Research Team has released an updated version of WorldWide Telescope — jammed packed with features including my favorite, 3D Solar System and Universe View (plug in your X-Box controller prior to running WWT for fully immersive 3D tranquility). If there was ever a way to grasp how little we really are, this will certainly help put things in perspective. You thought flying around in Virtual Earth 3D was cool? Wait until you fly around the known universe in 3D.
You'll need a 1GB RAM, Dual Core PC with a GPU with 256MB dedicated VRAM for best results. It will run on lower end hardware, but with slower frame rates and some visual issues.