Droom: Created by Nick D'Andrea
Droom is an open, explorative game creation ruleset. It has things in common with Zendo and Nomic, and probably shouldn't be attempted by novices. That said, I imagine that it is something that could be a lot of fun for thoughtful ludic gamers. Each player makes a game and plays out the game by rules of their own making--thus only knowing the winning condition for their own ruleset.
The archived "rules" are available here. here.