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

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

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
http://wordnet.princeton.edu/

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

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