Google has announced Google Sky, a new feature in Google Earth that lets you browse the entire night sky. Download the latest version of Google Earth, and you'll find a new button on the toolbar, which switches you to the sky view.
Sky mode includes clickable layers containing the complete Yale Bright Star Catalogue, Messier Catalogue and Dreyer's New General Catalogue of nebulae. Deep sky objects, such as the Horsehead Nebula, are accompanied by handy explanatory notes from Wikipedia. Other layers include a showcase of photos from the Hubble Space Telescope and a tour of stellar evolution, from star formation in a molecular cloud through to death as a planetary nebula.