Other
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.
The Write Place
Literacy Education Online: How to Use a Thesaurus
A good overview of what a thesaurus is and how best to use one. Also includes the difference between the two forms of thesauri and explains how to choose the correct synonym to use. L.9-10.4c References
University of Chicago
The University of Chicago: Roget's Thesaurus Search Form
Enter any word and get tons of words that mean the same thing (or close to it)
Other
Dictionary.com
Look up any word by typing the entry at the top. Includes word of the day, links to foreign language dictionaries, and link to a thesaurus. L.9-10.4c References, L.9-10.4d Verify meaning, L.11-12.4c References, L.9-10.4d Verify meaning,...
Other
The Phrase Finder
Use this tool to find the origins and meanings of phrases, sayings and cliches.
English Club
English Club: Learn English: Vocabulary: Topic Vocabulary Reference
Links to information and lists of English words and phrases by category. Includes information on the difference between a dictionary and thesaurus, synonyms and antonyms, confusing words, figures of speech, eponyms, euphemisms, sayings,...