I love it when fans get their creative juices flowing over their favorite properties. Karma Orange has a set of Doctor Who posters created with minimal information in them. I personally like the Cybermen poster most of all.
Now if only there were some way for advertising agencies to get in contact with these creators and pay them for their work, as opposed to out and out stealing their ideas and claiming them as their own. There have been a few times when a viral/fan project turned into something profitable for all parties, but those seem to be few and far between.