The Big Huge Thesaurus is a great service which has the capability to return synonyms, antonyms, related words, similar words and rhyming words (among other things). It can return JSON and XML and the pricing is very reasonable.
The gem itself has a very simple interface. You just use the
lookup method to lookup a word in the thesaurus.
Of course there is a convenience method called
Dinosaurus::synonyms_of which would get you the same results.