alphaDictionary
http;//www.alphadictionary.com/index.shtml/

Offers an amazing array of links to English and foreign language dictionaries as well as specialized dictionaries.

infoplease.com
http://www.infoplease.com/

Has been providing authoritative answers to all kinds of factual questions since 1938—first as a popular radio quiz show, then starting in 1947 as an annual almanac, and since 1998 on the Internet.

Thinkmap Visual Thesaurus
http://www.visualthesaurus.com/

An interactive dictionary and thesaurus which creates word maps that blossom with meanings and branch to related words.