Phrase Finder
- Enter a single word and it will return a list of phrases and sayings that are related to the word in some way.
- http://www.phrases.org.uk/

Thesaurus.com
- Searchable online version of Roget's Thesaurus of English words and phrases.
- http://thesaurus.reference.com/

Roget's International Thesaurus of English Words and Phrases
- 85,000 hyperlinked cross-references, plus proverbs and quotations from classic and modern authors.
- http://www.bartleby.com/110/

Mathematical Thesaurus
- Defines mathematical words and concepts, and indicates the links between them.
- http://thesaurus.maths.org/

Wordsmyth
- A dictionary and thesaurus in one. Also gives parts of speech and examples for more than 50,000 words.
- http://www.wordsmyth.net/

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

Roget's II - The New Thesaurus
- Contains 35,000 synonyms, succinct word definitions and a hyperlinked category index.
- http://www.bartleby.com/62/