Microsoft is taking on Google Sky and doing them one better with their new release of WorldWide Telescope. A "visualization software environment," the program turns your computer into a virtual telescope, with access to imagery from both ground and space-based telescopes.
You can choose a guided tour created by astronomers or educators, or explore on your own--or both. The program does have a hefty one hour download time, according to the website.
Unfortunately, the program is only for PC's at this point, but I hope Microsoft will eventually offer a Mac version. In the meantime, Mac owners who run Virtual PC (or Intel/Leopard users with BootCamp) can also use it.
How amazing will this be for science classes?!