Eventative.com is a tipping point based event planning website.
It's like Groupon meets Evites, where you plan any event (trip, party, poker game, reunion, rally, flash mob, etc.) that’s contingent upon enough guests signing up. The point is to avoid a lot of the back and forth hassle in the organizing process / potential cancellation and embarrassment if you are unable to acquire enough confirmed guests. You can also suggest events that are so crazy they otherwise might never have happened.