I think I just found the best free on-line dictionary at HyperDictionary.com. I used it twice so far today in solving the NYT crossword puzzles that are posted on Yahoo! In both cases they were words I had never heard of and both had definitions in Webster's 1913 dictionary.

I am curious if the 1913 Webster's dictionary is what HyperDictionary uses as its basis. I was also able to get the "WorldNet Dictionary" to come up on some subsequent searches.