Anyone know of a good prediction market / community?

I've had some success predicting world events, but nothing has come of it because I've utterly failed to identify secondary effects. Those secondary effects seem obvious in retrospect, but I never got there in my thinking. Is there a community of predictors who get together and bounce ideas off each other? I'm thinking specifically of the process described in "Scrum" where Dalkey and Helmer of Rand sought predictions from experts, but then analyzed the factors used in those predictions to repeat the process and achieve a high level of confidence. (Maybe the process is called a Delphi Exercise?) Not only would such a community help me refine practical decisions out of my predictions (and subject those predictions to rigorous challenge), but I suspect it'd also be a lot of fun.