An online thesaurus and dictionary of over 145,000 words that you explore using an interactive map. It's a tool for people who think visually. The most fun you've ever had with words. The Visual Thesaurus was built using Thinkmap, a data visualization technology... 

http://www.visualthesaurus.com/

Roget's II: The New Thesaurus - Find synonyms and word suggestions in this free online searchable thesaurus that features 260,000 synonyms and cross-references in an easy-to-use format, succinct word definitions, and an innovative hyperlinked category index on Yahoo! Education... 

http://education.yahoo.com/reference/thesaurus/

Wordsmyth. The Premier Educational Dictionary - Thesaurus. Free online dictionary with definitions written at three levels, synonyms, antonyms, examples, images, audio, semantic relations index. Look up any word. Create your own glossaries and quizzes. Use special tools to create and solve crosswords and solve anagrams. Reverse dictionary search in definitions, examples, synonyms and other fields... 

http://www.wordsmyth.net/

An on-line lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept. Different relations link the synonym sets. From the Cognitive Science Laboratory, Princeton University.. 

http://wordnet.princeton.edu/

Thesaurus.com - the largest and most trusted free online thesaurus brought to you by Dictionary.com. Quickly find synonyms and antonyms... 

http://thesaurus.com/

Free online dictionary, thesaurus, spanish-english and medical dictionaries, audio pronunciations, Word of the Day, word games, and many more high-quality Merriam-Webster language resources... 

http://www.merriam-webster.com/thesaurus.htm