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Beautiful Synonyms
Also offers antonyms and definitions.

Graph Words
Produces a color coded graph using data taken from the Wordnet lexicon for the chosen word, showing associations. Graphs can be saved and printed.

Merriam-Webster: Thesaurus
Provides synonyms, antonyms, related words and idiomatic phrases.

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.

Power Thesaurus
Also offers broader, narrower and related synonyms.
Includes word-related facts and word of the day.
Provides a searchable database and a partial word index to browse.
Online version of Roget's II: The New Thesaurus, 3rd edition.

Wordsmyth Educational Dictionary-Thesaurus
Integrated English dictionary and thesaurus.