Marble Hornets is one of the premiere Slender Man Web Series, although it's sometimes been called an ARG. All the information can be found in the "entries" as the videos are labeled. The basic rundown is that a kid named Alex was creating a movie named Marble Hornets. However, during the making of this video, his personality and demeanor began to change. He started filming everything he did instead of just making the movie. Alex eventually left all the video tapes he recorded with Jay, the owner of the youtube account that uploads all the "entries" with the command to simply burn the tapes. Something has been following Alex...
Most of all the updates for the ARG can be found on youtube channel: MarbleHornets.
In addition, another channel you should watch is totheark. You'll understand why.
Finally, the twitter page, which also updates frequently, can be found here: Marble Hornets Twitter