Roget's II - The New Thesaurus  
   Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index.
   http://www.bartleby.com/62/

WWWebster Thesaurus  
   Searchable online thesaurus from Merriam-Webster.
   http://www.m-w.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/

Mathematical Thesaurus  
   Defines mathematical words and concepts, and indicates the links between them.
   http://thesaurus.maths.org/

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/

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/