Mathematical Thesaurus [ Kids/Teens/Mature Teens ] - Defines mathematical words and concepts, and indicates the links between them. http://thesaurus.maths.org/

Phrase Finder [ Kids/Teens/Mature Teens ] - 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 [ Teens/Mature Teens ] - 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 [ Teens/Mature Teens ] - 85,000 hyperlinked cross-references, plus proverbs and quotations from classic and modern authors. http://www.bartleby.com/110/

Thesaurus.com [ Kids/Teens/Mature Teens ] - Searchable online version of Roget's Thesaurus of English words and phrases. http://thesaurus.reference.com/

Wordsmyth [ Kids/Teens/Mature Teens ] - 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 [ Kids/Teens/Mature Teens ] - Searchable online thesaurus from Merriam-Webster. http://www.m-w.com/thesaurus.htm