Other
Wordsmyth: The Educational Dictionary Thesaurus
This site not only is a dictionary and a thesaurus, but it also has brain teasers and games.
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Thesaurus.com: Homepage
To use the thesaurus at Thesaurus.com, simply type a word in the gold search box at the top of the page and click the 'Search' button. A list of synonyms and antonyms will be returned.
Other
The Phrase Finder
Use this tool to find the origins and meanings of phrases, sayings and cliches.