The Written World is a computer-mediated interactive storytelling game (additional details available here). The authors describe it as an interactive fiction MMO, but it’s also not completely unlike a library of Fiasco-style playsets.
The game provides assets — characters, character goals, possible events — embodying a story concept, but each actual experience is a two-player exchange between a Narrator player and a Hero player, a bit reminiscent of Sleep Is Death. The players participate primarily through writing, by creating descriptions of what happens next. If either of them doesn’t like what’s been done by the other, they can spend some Force to override the decision; Force is in turn earned by writing particularly compelling content. The aim of the exercise is not essentially competitive, but mediated cooperation aimed at producing an interesting story.
The Written World chief Simon Fox was kind enough to answer a few of my questions about how the mechanics work.
Emily Short's Interactive Storytelling 