Check out this site: Protagonize.com.
Try your hand at interactive fiction by visiting Protagonize.com, a new web site by Nick Bouton, a 29 year old software developer from Burnaby B.C.
You can choose from 18 different story categories and start writing a living, breathing narrative that may never end. Or, you can simply log on and contribute directly to the already growing number of stories started by amateurs, pros and semi-pros alike. And don’t be afraid of breaking continuity or taking the story in directions the original author intended, many of the stories are absurd or nonsensical.
Just don’t slay the main character. Bouton says it is the quickest way to kill a ripping yarn.