You can get Wordnet through the Wordnik API too ... as well as from the American Heritage Dictionary, The Century Dictionary, Wiktionary, and the GNU version of the Collaborative International Dictionary of English

And Wordnik is becoming a not-for-profit, if that makes a difference to you. :-)

Disclaimer: I am the founder of Wordnik. (You might like my TED talk: http://www.ted.com/talks/erin_mckean_redefines_the_dictionar...)

That said, I know the guy behind WordsAPI and he's good people. :-)