There are some really creative shoops of Slender Man into old photos and distorted film stills. Some great fan art, too.
One last note on Marble Hornets - if you have a couple free hours, I highly suggest watching the series. It's an ARG too, so keeping updated with the Twitter and the alternate "masky" account, really enhances the viewing experience (aka. helps to scare the shit out of you.) Watch it with the lights on, guys.