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Cartes du Ciel Sky Charts (freeware) - Win
Free program to draw sky charts on your very own desktop
http://www.stargazing.net/astropc/

Celestia (freeware) - Win/Mac/Linux
he free space simulation that lets you explore our universe in three dimensions. Celestia runs on Windows, Linux, and Mac OS X.
http://www.shatters.net/celestia/

NASA World Wind (freeware) - Win
World Wind lets you zoom from satellite altitude into any place on Earth.
http://worldwind.arc.nasa.gov/

Stellarium (open source) - Win/Mac/Linux
tellarium 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.
http://www.stellarium.org/

The Celestia Motherlode (freeware) - Win/Mac/Linux
The Celestia Motherlode is a repository for various addons like textures, models or celestial objects for Celestia.
http://www.celestiamotherlode.net/

Virtual Atlas of the Moon (freeware) - Win
Free software for Moon observation or survey for Windows systems only.
http://www.astrosurf.com/avl/UK_index.html