Bartleby.com: Reference Includes "Roget's International Thesaurus of English Words and Phrases" and Houghton-Mifflin's "Roget's II: The New Thesaurus," third edition. http://www.bartleby.com/reference/
Germ Searchable WordNet and Roget's Thesaurus with meaning trees. http://www.germ.cz/
Historical Thesaurus of English Contains the vocabulary of English from the earliest written records to the present, with known recorded dates of usage, taken from the Oxford English Dictionary. http://www2.arts.gla.ac.uk/SESLL/EngLang/thesaur/homepage.htm
INGRID Library Thesaurus Allows direct term search with results hyperlinked to broader and narrower search terms. Available in English and Estonian. http://ingrid.utlib.ee/thesaurus.html
Lexical Freenet Search for relationships between words and concepts that might never have occurred to you before. http://www.lexfn.com/
Phrases Thesaurus A resource for writers. Enter a single word and it will return a list of phrases and sayings that are related to the word in some way. In English. http://www.phrasefinder.co.uk
Plumb Design Visual Thesaurus An exploration of sense relationships within the English language. By clicking on words, you follow a thread of meaning, creating a spatial map of linguistic associations. http://www.visualthesaurus.com/
Roget's International Thesaurus of English Words and Phrases Online publication of this popular thesaurus made available by Bartleby.com. http://www.bartleby.com/110/
Roget's Thesaurus - As distributed by Project Gutenberg E-text of Roget's Thesaurus No. Two, which is derived from the version of Roget's Thesaurus published in 1911. http://asadz.com/thesaurus/
Sybrina's Phrase Thesaurus A creative phrase tool for creative writers of any genre including college students, people just learning English, people wanting to improve their communication skills, artistic professionals and others. http://www.sybrina.com
Wordsmyth Educational Dictionary-Thesaurus Integrated english dictionary and thesaurus. http://www.wordsmyth.net/