Dictionary.com
Search for any headword (the word being defined) contained in Webster's Revised Unabridged Dictionary or Roget's Thesaurus directly from this Web page. Just type in a word and click! This site also includes links to an on-line version of Bartlett's Familiar Quotations (1901 Edition) as well as to a few on-line dictionaries of computer terminology. Webster's Dictionary
An on-line version of Webster's Revised Unabridged Dictionary (1913 Edition); includes 109,562 entries. Perform simple "headword" searches or search the entire dictionary text. You can also perform floating (wild card) searches for words that contain strings of letters (e.g., prefixes or roots).
Roget's Thesaurus
Search the headwords or full text of Roget's Thesaurus (as supplemented, July 1991). Internal cross-references are represented as clickable hyper-text links.
RefDesk.com
An online resource that lets you choose among several online dictionaries with which to look up words. (Scroll down to find the dictionary list.)