WordNet - WordNet® is a large lexical database of English.
Glosbe - Huge collaborative massively multilingual dictionary.
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.
VerbAce - VerbAce-Pro is an easy-to-use Desktop Translation Software. Translate in a mouse click
Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...
Visuwords - The fun and free way to look up definitions. THE graphical dictionary and visual thesaurus.