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.shu.ac.uk/web-admin/phrases
Roget's II - The New Thesaurus - Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index. www.bartleby.com/62