Dictionary.Com
Free online English dictionary, thesaurus and reference guide, crossword puzzles and other word games, online translator and Word of the D
http://www.dictionary.com

English thesaurus
The synonyms dictionary lists more than 30,000 words with their synonyms. It is a browsable database of synonyms and related words.
http://www.synonyms.ca

Getty Thesaurus of Geographic Names (Research at the Getty)
Getty Thesaurus of Geographic Names (TGN) is a structured vocabulary of geographic names for indexing art and architecture.
http://www.getty.edu/research/tools/vocabulary/tgn/

Merriam-Webster Online
A free, searchable on-line dictionary and thesaurus, word games, a word of the
http://www.m-w.com

Roget's Thesaurus
Free online Roget's Thesaurus of the English Language.
http://www.thesaurus.com

Thinkmap Visual Thesaurus
An online thesaurus and dictionary of over 145000 words that you explore using an interactive map. It's a tool for people who think visually.
http://www.visualthesaurus.com