Stellarium is free GPL software which renders realistic skies in real time with openGL. With Stellarium, you really see what you can see with your eyes, binoculars or a small telescope. Stellarium is also used in real planetariums.
|From Linux and Lamp – Open Source Software|
Works with Linux, Mac OSX and Windows. There is also one that you can carry along with you – Stellarium at PortableApps