The granddaddy of most online horror series, Marble Hornets opens with Jay inheriting unedited footage of a friend's student film. Curious about what happened to cancel filming, he starts watching them and finds something terrible lurking behind the scenes. Armed with a box of tapes and a cast list, Jay sets out to find out what the hell happened to everybody who was a part of Marble Hornets.
The first major Slender Man series, Marble Hornets documents a descent into madness, misery and murder, with an on-going mystery that enthralled viewers for three years, spanning two YouTube channels and two Twitter accounts, and even got praise from Roger Ebert. You can check it out here.