American Society of Indexers - Thesauri Online
   Annotated directory.
   http://www.asindexing.org/site/thesonet.shtml

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/

HASSET
   Provides direct term search followed by hierarchical and other types of browsing. Based on a UNESCO thesaurus.
   http://www.data-archive.ac.uk/search/hassetSearch.asp

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 Interactive Thesaurus
   Roget's Thesaurus of English words and phrases.
   http://thesaurus.reference.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

WWWebster Thesaurus
   Merriam-Webster Thesaurus
   http://www.m-w.com/thesaurus.htm

WordNet - a Lexical Database for English
   An on-line lexical reference system whose design is inspired by current psycholinguistic theories of human lexical memory. English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept. Diff
   http://www.cogsci.princeton.edu/~wn

Wordsmyth Educational Dictionary-Thesaurus
   Integrated english dictionary and thesaurus.
   http://www.wordsmyth.net/

Yahoo! Roget's II Thesaurus
   Contains over 260,000 synonyms and cross-references. Features succinct word definitions and hyperlinked category index.
   http://education.yahoo.com/reference/thesaurus