The Shortlist has a pretty nifty gallery of 12 Back to the Future posters as interpreted by indie artists - my favorite two are shown above, the one to the left is created by Phantom City Creative in collaboration with Twitch Film and the TIFF Bell Lightbox and the one to the right is in Saul Bass-style by Dave Will (don't miss his BTTF2 poster as well): http://www.shortlist.com/entertainment/article/1200/12-back-to-the-future-posters - Thanks Benjamin!
See also: Back to the Future 2 DeLorean Time Machine | Bleak is the Future