I think I saw this on Boing Boing a few weeks back. Maybe you guys saw it there too. In case you didn't I'm mentioning it here.
There's this novel being written online in which the author writes about a chapter a week and at the end of each chapter there is a choice to be made. Once you read it you vote on which choice you prefer and the next week the choice with the most votes is where the author picks up for the next chapter. I've always thought a wikifiction concept would be cool. It turns out they tried it already and it didn't work out as well as I would have hoped. This idea is less free form and so far the book is pretty good. It seems to be well within the category "zombie book." Anyway, if you like choose your own adventure books, like dark stories and like zombies you should go read this from the start so you can keep up as it gets longer.
p.s. They call it a "wovel" which I think is a stupid name.