Might want to double check the license, but the baseline standard is the CMU Pronunciation dictionary, which is freely downloadable and also ships with many NLP libraries, like NLTK (python).
http://www.speech.cs.cmu.edu/cgi-bin/cmudict
For out-of-vocabulary words, I've had great success with Sequitur G2P, which is both trainable and under the GPL:
http://www-i6.informatik.rwth-aachen.de/web/Software/g2p.html
edit: note that CMUDict (and many other speech processing pipelines) represent pronunciation in ARPAbet. I apparently don't have enough points to post more links, but google "FAVE ARPABET" and you'll get a handy cheat sheet.