Reference -> Dictioneries

• Encyclopaedia Britannica
  Headquartered in Chicago, Illinois, are leading providers of learning and knowledge products.
  http://www.brittanica.com
  
• Encyclopedia
  The Internet's premiere free encyclopedia, provides users with more than 57,000 frequently updated articles from the Columbia Encyclopedia.
  http://www.encyclopedia.com
  
• Online Dictionary
  Information Please is the world's largest free reference site.
  http://www.infoplease.com