Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up, passive desktop notifications, regular expression based search, etc.. Artha may be used as a free open-source replacement to the proprietary WordWeb Pro.
Artha passively notifying a selected text's definition
Regular Expression based search on Artha, using wildcard (*)
Detailed Antonyms of 'rich'
Detailed mode ('Parts' [meronyms] of car)
Domain Terms of 'Photography'
'water' is a kind of... (Hypernyms - Detailed Mode)