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; the interface, however, is easy to use, and it allows you to quickly navigate to many interesting points in the night sky you'd never be able to see standing outside with your naked eye.
The interface won't take any time to get used to, either. The project uses Google Maps to drive its navigation. You can pan, zoom, and explore the night sky the same way you would Manhattan. If using Star Viewer has you interested in checking out more astronomy-related tools, we'd highly recommend checking out the aforementioned Stellarium as well as another previously mentioned web-based tool, Microsoft's WorldWide Telescope.
Have your own favourite site, application or tool for exploring the night sky? Let's hear about it in the comments.