Mathematical Thesaurus: Defines mathematical words and concepts, and indicates the links between them.
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.
Roget's II - The New Thesaurus: Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index.
Roget's International Thesaurus of English Words and Phrases: 85,000 hyperlinked cross-references, plus proverbs and quotations from classic and modern authors.
Thesaurus.com: Searchable online version of Roget's Thesaurus of English words and phrases.
Wordsmyth: A dictionary and thesaurus in one. Also gives parts of speech and examples for more than 50,000 words.
WWWebster Thesaurus: Searchable online thesaurus from Merriam-Webster.
