educational (3)
Celestia
http://www.shatters.net/celestia
Educational software for gazing upon the stars. Check out the Celestial Motherlode too. (www.celestiamotherlode.net)
Google Earth
http://earth.google.com/
If you don't know what this is yet, you've been under a rock in the middle of Australia. Check out your house with this.
OmniDictionary
http://www.omnigroup.com/applications/omnidictionary/
A free mac OS X client for Internet Dictionary servers. A nice, concise application which does away with the need for a hard copy of a dictionary.