Dictionary.com
http://www.dictionary.com/
Accesses New Hackers, Skeptic, Websters. etc.

Merriam Webster Collegiate Dictionary & Thesaurus
http://www.m-w.com/

Roget's Thesaurus of English Words and Phrases
http://www.thesaurus.com/
Online version of the old standby

Semantic Rhyming Dictionary
http://www.link.cs.cmu.edu/dough/rhyme-doc.html
Online rhyming dictionary. You enter the word, it finds words that rhyme with it. 

Definitions
http://www.dict.org/bin/Dict
Definitions Search

Wordsmyth
http://www.wordsmyth.net/
Advanced Synonym Search

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