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.
Germ - Searchable WordNet and Roget's Thesaurus with meaning trees.
HASSET - Provides direct term search followed by hierarchical and other types of browsing. Based on a UNESCO thesaurus.
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.
INGRID Library Thesaurus - Allows direct term search with results hyperlinked to broader and narrower search terms. Available in English and Estonian.
Lexical Freenet - Search for relationships between words and concepts that might never have occurred to you before.
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.
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.
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.
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