Germ
Searchable WordNet and Roget's Thesaurus with meaning trees.
http://www.germ.cz/

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

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/

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/

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

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

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

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/

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

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

Lexical Freenet
Search for relationships between words and concepts that might never have occurred to you before.
http://www.lexfn.com/

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

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

Thinkmap Visual Thesaurus
Interactive dictionary and thesaurus using a visual display for data navigation.
http://www.visualthesaurus.com/

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. Different relations link the synonym sets. From the Cognitive Science Laboratory, Princeton University
http://wordnet.princeton.edu/

Online Thesaurus
Find synonyms using this free online thesaurus from AOL's Research and Learn channel.
http://reference.aol.com/thesaurus

WWWebster Thesaurus
Merriam-Webster Thesaurus
http://www.merriam-webster.com/thesaurus.htm