Wordnet for Mac OS X 10.1 and later, is a freeware lexical reference system (a dictionary and much much more). It includes spellchecking, synonyms, antonyms, and a hypernym browser (e.g. cat ? feline ? … ? mammal ? … ? animal ? … ? living thing ? object ? entity) developed at Princeton University’s Cognitive Science Laboratory under the direction of George Miller, James S. McDonnell, Distinguished University Professor of Psychology, Emeritus. More info and download link here.