'The Great Game' is a work of interactive fiction, a particpatory story where you're in charge of what happens next. I write a bit. You tell me what happens next, then I go off and write that, and give you your next choice.
The Great Game is updated twice a week, on Mondays and Thursday, a few hours after voting closes for the previous installment.
The Great Game is written by Tim Dedopulos and illustrated randomly by generous Creative Commons folk on Flickr and DeviantArt. The original idea for this type of interactive story-telling comes from tabletop RPG design genius Robin Laws.
Tim's main blog is Ghostwoods, where he talks about fiction, myth, writing, strange bits of reality, vampires, and other bits and pieces. He also twitters as Ghostwoods, and answers any questions put to him -- no matter how random -- at Formspring.
If you like the Great Game, you'll want to check out Justin Achilli's participatory fiction story, Hell Harbor. It has pirates!
You should also drop in on Eddy Webb's atmospheric Whitechapel Project, about an enigmatic prisoner.
Grim of Postmortem Studios is running a game too, Zombie. It's starting off in a school!