http://www.bartleby.com/
Bartleby publishes thousands of free online classics of reference, literature, and nonfiction.

http://www.merriam-webster.com/
Merriam-Webster is America's foremost publisher of language-related reference works.  The company publishes a diverse array of print and electronic products, including Merriam-Webster's Collegiate® Dictionary, Eleventh Edition -- America's best-selling desk dictionary -- and Webster's Third New International Dictionary, Unabridged.

http://humanities.uchicago.edu/orgs/ARTFL/forms_unrest/ROGET.html
The ARTFL Project is a consortium-based service that provides access to North America's largest collection of digitized French resources.

http://www.onelook.com/
A search engine for words and phrases with more than 5 million words in more than 900 online dictionaries that are indexed by the OneLook® search engine.

http://www.bartleby.com/141/
Intended for use in which the practice of composition is combined with the study of literature, it gives in brief space the principal requirements of plain English style and concentrates attention on the rules of usage and principles of composition most commonly violated.

http://commtechlab.msu.edu/sites/aslweb/browser.htm
Dictionary for American Sign Language with illustrations.

http://www.rhymezone.com/
RhymeZone is an online tool that aspires to be the best and fastest way for writers to find words.