Stellarium is free GPL software which renders realistic skies in real time with OpenGL. With Stellarium, you truly see what you cannot see with just your eyes, binoculars, or a small telescope. Stellarium is used also in planetariums.
Review System Requirements
Download from Stellarium
Stellarium: Frequently Asked Questions
Stellarium: User's Guide
Stellarium: Wiki
Referenced from: Stellarium.org