WorldWide Telescope "... it used to be a Windows-only desktop application. Now, provided you're running an operating system that supports Microsoft's Silverlight browser plug-in, you can enjoy the same star-gazing fun from any browser." (link)
"WorldWide Telescope (WWT) enables your computer to function as a virtual telescope, bringing together imagery from the best ground and space-based telescopes in the world. Experience narrated guided tours from astronomers and educators featuring interesting places in the sky." (link)Watch the TED talk about the WorldWide Telescope:
Stellarium "Windows/Mac/Linux: Whether you're a die-hard astronomy buff or someone who'd just like an idea of what constellations are where, Stellarium is a fantastic tool for viewing the night sky from the comfort of your home. ...At its most basic, Stellarium will display the night sky as seen from anywhere on earth. Delving into the more advanced features you can do all sorts of really interesting things like see the constellations for a dozen different cultures—the Pegasus from Greek mythology is the Turtle in the Navajo tradition." (link)
photo credit: poulz
"Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
It is being used in planetarium projectors. Just set your coordinates and go." (link)
Watch this video from YouTube created with Stellarium:
Celestia is a free space simulation that lets you explore our universe in three dimensions. Celestia runs on Windows, Linux, and Mac OS X. Celestia doesn't confine you to the surface of the Earth. You can travel throughout the solar system, to any of over 100,000 stars, or even beyond the galaxy.
"Looking down at Mars's tiny moon Phobos and the giant Valles Marineris rift valley." photo link
This video shows some of the features of Celestia:
You'll find an extensive list of astronomy software at http://www.midnightkite.com/software.html