Celestia is an application for real-time 3D visualization of space, with a detailed model of the solar system, over 100,000 stars, more than 10,000 galaxies, and an extension mechanism for adding more objects.
I love this planetarium software. I turn it on before going to bed and watch a real time view of places in the solar system while I drift off to sleep. I do wish they'd update it for modern machines as there've been no new versions for awhile. I'd like a 64 bit version to maximize my computational resources.