Stellarium is a free open-source program simulating a planetarium, available for Linux, Mac OS X and Windows. Its features, among these important, are e.g. a default catalogue of over 600,000 stars (and an extra catalogue with more than 210 million stars), planets and their satellites, a powerful zoom, time control, multilingual interface, telescope control, various visualisation options, customisability ASO.
When you are a bit tired of entertainment industry programs (commonly knows as games ;-)), you can take a glance at Stellarium.
The official site: Stellarium.org
SourceForge site: Sourceforge.net
This is BRILLIANT - now all I need is a giant dome and a projector... Great idea guys.