Any of Ben Robbins' games: Microscope and its variants Chronicle and Union, Kingdom, and Follow.
Microscope is very different from other RPGs: You don't have a specific character, and you're creating an entire history. It's great for creating a setting together for another game! It can be played for as long or short a time as you please. (Chronicle is a variant that describes the history of some specific thing instead of an entire setting; Union describes a family tree.)
Kingdom is a more traditional game in which you're deciding the fate of a "kingdom" - which can be any organized group of people with a common end. (ie, a street gang or a business firm qualify as easily as a country.) Characters take on the roles of Power (you make the decisions), Perspective (you decide the fallout of decisions), and Touchstone (you decide how the rank-and-file react). Your character's role can change over the course of the game. It's a blast!
Follow is a single session quest game - it's inherently a one-shot. You are a band of characters trying to achieve a specific goal. You have both a primary and a secondary character, because attrition is pretty fierce! My group has had great fun with this one too.
