A really interesting site I found while looking around, is Alternative Movie Posters. It's collection of various remakes of classic movie posters, in some really different styles that are all created by just regular people. So some are a bit lacking in quality, but for the most part there are some very interesting takes on poster designs.
That aside, there are many other really interesting posters I've found on that site. Of those I've found, my favorites tend to be the very simplified sort of style.