Math.com

Great source for all things math---homework help, practice, online calculators and games. 

Fact Monster

Provides homework help in all major subject areas, as well as an online dictionary, encyclopedia, thesaurus, almanac, atlas and timelines.

Homework Helper Refdesk.com 

A great resource, broken down by grade level and subject matter. Lots of links to great sites.

Refdesk.com

"The single best source for facts" and current events information.