Celestia is a 3D astronomy program. The program allows the user to travel through an extensive and accurate universe at any speed, in any direction and at any time in history (GPL)

ORSA is an interactive tool for scientific grade Celestial Mechanics computations. Asteroids, comets, artificial satellites, Solar and extra-Solar planetary systems can be accurately reproduced, simulated, and analyzed. (GPL)

Stellarium 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. (GPL)