Aciqra is a free and open source virtual planetarium and sky mapping program which tracks celestial bodies including planets, deep sky objects and stars.
Downloads: 47744 - Freeware - Download Now

Educational orbit and gravitation simulator for high school, college and beginning university physics and astronomy. Easily create, run, print and save a simulation. Includes 29 premade simulations with activities page in html-format.
Downloads: 47509 - Shareware - Download Now

Gravit is a gravity simulator which runs under Linux, Windows and Mac OS X. It uses Newtonian physics. Although the main goal of Gravit is to be as accurate as possible, it also creates beautiful looking gravity patterns.
Downloads: 47188 - Freeware - Download Now

Twelve months in the making and Solar Kingdom has now been released.
Downloads: 48480 - Demo - Download Now

Accurate, flexible wireframe simulator for the Voyager and Giotto space missions, with animation capability. Developed at JPL for science/navigation, and filmed by the BBC/NOVA. Conveniently runs from any Windows/DOS.
Downloads: 47706 - Freeware - Download Now