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.
Glosbe - Huge collaborative massively multilingual dictionary.
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.
WordNet - WordNet® is a large lexical database of English.
Wordnik - the biggest online English dictionary, by number of words