WordNet - WordNet® is a large lexical database of English.
Power Thesaurus - Power Thesaurus is a fast, comprehensive and easy-to-follow online thesaurus for writers...
Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...
GoldenDict - The program has the following features: Use of WebKit for an accurate articles' representation, complete with all formatting, colors, images and links.
Moby Thesaurus - 30,000 root words, 2.5 million synonyms andrelated words
WordWeb - One-click lookup in any almost any Windows program; Hundreds of thousands of definitions and synonyms; The latest international English words; Works offline, or reference to Wikipedia and web references.