Thesauri

Mathematical Thesaurus :

[ Kids/Teens/Mature Teens ] - Defines mathematical words and concepts, and indicates the links between them.
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.
Roget's II - The New Thesaurus :

[ Teens/Mature Teens ] - Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index.
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.
Thesaurus.com :

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

[ Kids/Teens/Mature Teens ] - A dictionary and thesaurus in one. Also gives parts of speech and examples for more than 50,000 words.
WWWebster Thesaurus :

[ Kids/Teens/Mature Teens ] - Searchable online thesaurus from Merriam-Webster.