Celestia: visualize the Milky Way on your PC

Celestia is open-source software (under the GNU GPL license) for real-time 3D simulation of space, which is not limited to Earth but to the entire Milky Way.
 
Programmed in OpenGL (100,000 lines of code), this software which runs on Windows, Linux and Mac OS, is the work of four programmers. Making extensive use of bump-mapping to draw the relief of the surface of the planets, the program allows free movement in space, and really impresses with the notion of scale that it makes clear (the rendering of each planet taken separately not being the most impressive compared to other simulations). It is based on NASA data, guaranteeing a real simulation. A software you absolutely must try!