Stellarium is free GPL software which renders realistic skies in real
time. With Stellarium, you really see what you can see with your eyes,
binoculars or a small telescope. Clicking on a star, planet or constellation provides some information about it such as distance from the earth, brightness, etc.
The free space simulation that lets you explore our universe in three
dimensions.
Celestia
runs on Windows, Linux, and Mac OS X.
