Thesauri

Chambers Reference Online
Includes Chambers Thesaurus, Chambers 21st Century Dictionary and Chambers Biographical Dictionary.

Merriam-Webster Online
Online thesaurus. 

Roget's Thesauri
Online access to Roget’s Thesauri, Roget's II, and Roget's International Thesaurus of English Words and Phrases via Bartleby.com