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/

Wordsmyth
A dictionary and thesaurus in one. Also gives parts of speech and examples for more than 50,000 words.
http://www.wordsmyth.net/

WWWebster Thesaurus
Searchable online thesaurus from Merriam-Webster.
http://www.merriam-webster.com/thesaurus.htm