Microsoft WorldWide Telescope
WWT 's clean interface gets you out there faster than you can say "Sputnik." The "Look At" picklist gives you your choice of Earth, Panorama, Planet, and Sky. It takes a setting tweak to make Earth's street maps clear enough to be useful, and Planets consists of three real planets, five moons, and a model Mandelbrot fractal planet--but no matter. Microsoft might have done better to name this OffWorldWide Telescope, because its strength lies in the stars. The few panoramas are attractive, but oh, the Sky is gorgeous.
The Explore menu opens the universe to you in easily understood collections: Constellations, Solar System, Hubble Images, Named Stars, and so forth. Just click a collection and scroll through the thumbnails until you find a spot you'd like to explore. I saw a few blocky artifacts while the camera moved, but overall, the images were breathtaking. In the downloadable Guided Tours, astronomy experts talk you through brief explanations of the images on your screen.
Whether you're a serious stargazer or just a tourist, Microsoft WorldWide Telescope puts all your earthly worries into perspective.
Note: This link takes you to the vendor's site, where you can download the latest version of the software. This is beta software, and its stability is not guaranteed. Use at your own risk.