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. It is being used in planetarium projectors because it is a free software planetarium, licensed under the terms of the GNU General Public License, available for Linux, Windows and Mac OS X. It uses OpenGL to render a realistic sky in real time. With Stellarium, it's possible to see what one cannot see with the naked eye, binoculars or a small telescope. This is a very nice tool to use with your students in classroom and in night-time observations preparation. One can see past, present and future events like eclipses, Venus and Mercury transits or even reproduce Galileo's observations about Jupiter's moons.
A screenshot of Stellarium
Stellarium is developed by the French programmer Fabien Chéreau, who launched the project in the summer of 2001. Other developers include Robert Spearman, Johannes Gajdosik, Matthew Gates, Nigel Kerr and Johan Meuris. John Meuris is responsible for most of the the artwork done with the constellations and landscapes.
Stellarium was featured on SourceForge in May 2006 as Project of the Month.