Following Bill's excellent presentation on Astronomy Software at our January CAS Meeting, here are links to some of the software demonstrated;
Stellarium [Planetarium] - http://www.stellarium.org
WinStars2 [Planetarium and simulator] - http://www.winstars.net
Astronomy Lab [Ephemeris] - http://www.personalmicrocosms.com/Pages/ss_alw32.aspx
Celestia [space simulator] - http://www.shatters.net/celestia/
NASA World Wind [Planet models] - http://worldwind.arc.nasa.gov/
SkyGlobe [DOS based planetarium] - http://www.sidewalkastronomy.com/skyglobe.html
As Bill mentioned all are available for free download.