Thesauri

» Mathematical Thesaurus
   Defines mathematical words and concepts, and indicates the links between them.

   
   http://thesaurus.maths.org/

» Phrase Finder
   Enter a single word and it will return a list of phrases and sayings that are related to the word in some way.

   
   http://www.phrases.org.uk/

» Roget's II - The New Thesaurus
   Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index.

   
   http://www.bartleby.com/62/

» Roget's International Thesaurus of English Words and Phrases
   85,000 hyperlinked cross-references, plus proverbs and quotations from classic and modern authors.

   
   http://www.bartleby.com/110/

» Thesaurus.com
   Searchable online version of Roget's Thesaurus of English words and phrases.

   
   http://thesaurus.reference.com/

» WWWebster Thesaurus
   Searchable online thesaurus from Merriam-Webster.

   
   http://www.merriam-webster.com/thesaurus.htm

» Wordsmyth
   A dictionary and thesaurus in one. Also gives parts of speech and examples for more than 50,000 words.

   
   http://www.wordsmyth.net/