Ruby-WordNet sounds like it does what you're looking for:
Ruby-WordNet is a Ruby interface to
the WordNet® Lexical Database. WordNet
is an online lexical reference system
whose design is inspired by current
psycholinguistic theories of human
lexical memory. English nouns, verbs,
adjectives and adverbs are organized
into synonym sets, each representing
one underlying lexical concept.
Different relations link the synonym
sets.
