Merriam-Websters has gone Wiki (sort of). You can now submit your own words to their "open dictionary."
Submission guidelines are what you'd expect and MW reserves the right to delete the work of mean-spirited idiots or spammers (my words, not theirs).
I think this could turn out pretty cool, as long as MW does a good job of weeding out crap and redundancy. It would also be nice to be able to search the text of definitions, as well as the words. It's nice that they let you browse by category (anatomy, business, food, language, etc.)