One of the most widely known ARGs is the horror/thriller game Marble Hornets (although, it is often seen as more of a chaotic fiction). MH tells the story of a group of friends who are haunted by the ghostly figure of Slenderman, as well as a number of other unsettling characters. The story revolves around the character Jay who is given a bin bag full of old videos by his friend Alex and told to destroy them. Instead Jay watches the tapes, originally meant to be film from the budget movie they and a few friends were filming, and sees Alex's sanity unwind as his is stalked by Slendy.
The story is mostly told through youtube and Twitter (@marblehornets) posts, with updates coming in the form of "Entries" - beginning with The Introduction and culminating in Entry 26. I don't want to give too much of the story away, just in case you are interested in getting involved, but its certainly worth watching from the start - also keep an eye on the video responses!
The main reason for this post is that "Season 2" appears to be starting, with Jay's Twitter being updated with 3 new mysterious and cryptic pictures - the ARG forums are furiously attempting to decode them, check out the action on unFiction here. I would thoroughly suggest getting involved in the forum and contributing, they are all very friendly and the community spirit of solving each problem as a group is really rather thrilling. There are tonnes of other viral and grassroots ARGs to get involved in on the unfiction forums, it was a big part of my life in the early part of this year and I've lost contact with the group of late, I'm hoping to get back into it with the new season of Marble Hornets.
Just.. erm, watch your windows, do not start to film yourself and, whatever you do, don't go into the freaking woods...