Probably old news to some of you, but I'd never heard of it before; Celestia is an open-source astronomy program that tries to do for astronomy what Google Earth does for geography. Zoom in on planets and speed up their rotation, zoom out and see the galaxy, etc. etc. There are versions for Windows, Mac OS-X and Linux, and you can download the source code, also lots of add-ons available, an educational version, etc.
The Windows version is a 16mb download, just about runs on the 400mhz XP box I use at work and is like greased lightning on a 2 ghz machine. Amazingly pretty, and very strongly recommended