Microcosm

From publisher's website: Microcosm is a very light set of rules for tabletop play. The creation of situations is treated as part of the act of play. This set of rules was designed as an introductory game, for use by tabletop roleplayers to show their friends what their hobby is all about. It excels at one off, no preparation play, but is not intended for high detail or long term campaigning.