If you've never abandoned your 4th grade obsession with the night sky (and who has?), Star Viewer is a web-based tool for peeking at some of the most interesting and vivid sights in the night sky. Star Viewer is not as complicated as, say, the previously mentioned open-source and feature-rich astronomy tool Stellarium
Desktop Earth is a wallpaper generator for Windows. It runs whenever you're logged on and updates your wallpaper with an accurate representation of the Earth as it would be seen from space at that precise moment.