Reach out for the stars with WWT

If you are fond of astronomy, you shouldn't miss Microsoft Research's WorldWide Telescope (WWT)

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

There is a Web-based version running on Silverlight and a Windows Client version to choose from.