Oxford English Dictionary
Just loaded OS X Lion and discovered that my Merriam Webster dictionary was not compatible with this software, so I purchased the OED. Its a fine dictionary based on outstanding lexicography. But the app description should explicitly note that the audio pronunciations are those of British speakers, not American. Thus, schedule is pronounced "shedule" and lieutenant, "leftenant." Not a deal breaker, but something buyers should know up front.
Wroodiii about Oxford Dictionary of English