Celestia is an open source space simulator, based on OpenGL. Using it, you can view virtually any celestial object in 3D, in realtime. It makes for a great toy and some excellent eye candy, you can spend hours just clicking around, watching Earth spin, seeing all the planets, marvelling at the natural beauty of the universe...oh and it's educational too, I guess.

It runs relatively fast at low resolutions (my TNT2 is chucking out plenty of FPS) and isn't very CPU intensive. There are KDE and GTK2 frontends available, and the program itself is available either through APT (RPM and Debian) and from the program's homepage at http://www.shatters.net/celestia/. There is a version available for Windows, as well as Linux.