Thesauri

Mathematical Thesaurus - Defines mathematical words and concepts, and indicates the links between them.
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.
www.phrases.org.uk/

Roget's II - The New Thesaurus - Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index.
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.
www.bartleby.com/110/

Thesaurus.com - Searchable online version of Roget's Thesaurus of English words and phrases.
thesaurus.reference.com/

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

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