Celestia is a free software package that graphically simulates celestial objects ranging from the earth and our solar system to distant constellations and galaxies. Simulating motions, light and shadow in three dimensions, the program generates amazing 3D images and allows the user to travel to any place in the universe and visit planets, comets, and other objects in the solar system.
You can follow the paths of satellites and space probes and observe to numerous stars and galaxies. It is based on the Hipparcos Catalogue and represents more than 100,000 stars and 10,000 galaxies. The software supports multiple star systems and uses advanced technologies for shading.
Celestia can be extended for example by adding high resolution images of planet surfaces. The program is licensed under GPL and can be downloaded here. Versions are available for Linux, Windows, and Mac OS X.
Another cool planetarium software project is Digital Universe/Partiview, available here.