Celestia
(Version 1.4.1)

Celestia is a simulation of the entire universe, based on current astronomical information. It includes beautifully rendered planets, moons, and some smaller orbital bodies such as Mir and the International Space Station. Celestia is not limited to our solar system; you can visit other stars, or even travel outside the galaxy. You will find models of extra-solar planets in orbit around those stars for which we have evidence of planetary bodies. You have complete control over time and space, so you can view the stars from your chosen location and epoch. In addition, it is possible to create scripted presentations for complex demonstrations.

This program is highly recommended for use in school presentations. There are some lesson plans for use in a middle/high school setting, contributed by Frank Gregorio . To get a good idea of Celestia's full capabilities, start the program and press D for Demo. Celestia requires an OpenGL-capable graphics card (which nearly all modern systems have). It is available for Windows, Linux, and MacOS X.