I downloaded this yesterday and played around a little bit with it. It's quite awesome to explore the stars in this fashion. What is it? Well, to quote the description from the website:
"The WorldWide Telescope (WWT) is a Web 2.0 visualization software environment that enables your computer to function as a virtual telescope—bringing together imagery from the best ground and space-based telescopes in the world for a seamless exploration of the universe. Choose from a growing number of guided tours of the sky by astronomers and educators from some of the most famous observatories and planetariums in the country. Feel free at any time to pause the tour, explore on your own (with multiple information sources for objects at your fingertips), and rejoin the tour where you left off. Join Harvard Astronomer Alyssa Goodman on a journey showing how dust in the Milky Way Galaxy condenses into stars and planets. Take a tour with University of Chicago Cosmologist Mike Gladders two billion years into the past to see a gravitational lens bending the light from galaxies allowing you to see billions more years into the past. WorldWide Telescope is created with the Microsoft® high performance Visual Experience Engine™ and allows seamless panning and zooming around the night sky, planets, and image environments. View the sky from multiple wavelengths: See the x-ray view of the sky and zoom into bright radiation clouds, and then crossfade into the visible light view and discover the cloud remnants of a supernova explosion from a thousand years ago. Switch to the Hydrogen Alpha view to see the distribution and illumination of massive primordial hydrogen cloud structures lit up by the high energy radiation coming from nearby stars in the Milky Way. These are just two of many different ways to reveal the hidden structures in the universe with the WorldWide Telescope. Seamlessly pan and zoom from aerial views of the Moon and selected planets, as well as see their precise positions in the sky from any location on Earth and any time in the past or future with the Microsoft Visual Experience Engine. WWT is a single rich application portal that blends terabytes of images, information, and stories from multiple sources over the Internet into a seamless, immersive, rich media experience. Kids of all ages will feel empowered to explore and understand the universe with its simple and powerful user interface. Microsoft Research is dedicating WorldWide Telescope to the memory of Jim Gray and is releasing WWT as a free resource to the astronomy and education communities with the hope that it will inspire and empower people to explore and understand the universe like never before. "