Encyclopedias
Britannica Concise - http://concise.britannica.com/
A concise version of the Encyclopedia Britannica. Has around 28,000 articles online.
Encyclopedia Smithsonian - http://www.si.edu/Encyclopedia_SI/
Smithsonian resource site. Offers answers for questions ranging from A to Z and can be searched by keyword.
PlanetMath - http://planetmath.org/
A math encyclopedia that features articles that have been peer reviewed.