Ducksters:
http://www.ducksters.com/
Fact Monster:
http://www.factmonster.com/
InfoPlease:
http://www.infoplease.com/
RefDesk.com:
http://www.refdesk.com/
LibrarySpot.com:
http://libraryspot.com/
Times for Kids:
http://www.timeforkids.com/