Thesauri

Thesaurus.com
A great online thesaurus which offers definitions, parts of speech, and links to all synonyms.
http://thesaurus.reference.com/
Merriam-Webster Online
Merriam-Webster provides a free online dictionary, thesaurus, audio pronunciations, Word of the Day, word games, and other English language resources.
http://www.m-w.com/thesaurus.htm