And now for something completely different... Microscope.
Microscope is less a roleplaying game than a collective worldbuilding game. A round-table non-linear game of free-style history building. In Microscope, players don't have player-characters, character sheets or even dice. You have a stack of 3x5 notecards and pencils and that's it.
Let me backup and explain that again.
As a group you begin by discussing in broad strokes what your setting and history is going to be about. You define the tropes that are and aren't included in that universe. You define the beginning and end points of your history (the 'bookends'). Then you take turns going around the table, adding details both large and small to your collaborative history. The player who's turn it is has complete narrative control over what goes on, generally, so long as they don't contradict the established tropes and go beyond the scope of your history bookends. It's a completely non-linear process: On one turn a player creates a fantastic city. On the next turn someone else destroys it. For the rest of the game all players can continue to add events involving that city between it's creation and destruction. All Periods, Events and Scenes have relative order, but not explicit order (you don't assign dates to your events except as color to the narrative). If one player's input seems to derail the 'story,' there's no limit to how to bring it back on track.
The three levels of playable details are: Periods or broad history (think "The Bronze Age", or "Man's first explorations of space"), followed by Events ("The Tribes migrate to the Rivers," "The Federation and The Empire war over the Delta Sector."). Events consist of Scenes ("The Emperor confronts the Senate over Solarian slavery," "The Hero Galoka befriends the Huluzian tripartate"). You cannot play a Scene before it's encompassing Event and you can't play an Event without a Period to place it in. There's no limit to the number of Periods in your history, or the number of Events in a Period, or Scenes in an Event. Game ends either when the allotted play time has passed, or players are satisfied with what they have accomplished.
Microscope is amazingly simple, yet fundamentally deep and awesome in it's potential. It's rulebook (as such) is small and easily portable and is also available in PDF form which reads well on tablets (I have a Nook color).
While one can play Microscope entirely by its self, I think one of it's greatest potential uses is for worldbuilding for other games. Worldbuilding is best done collectively, which gives all involved players investment and involvement in the setting they are going to play in. The only change to Microscope is that when you get down to the Event and Scene level, that's when you break out your other game books, roll up characters, and play the resolution to that Event or Scene.
Microscope is just the thing for creating organic, collaborative settings and histories that have player investment and the potential for depth and inspiration to play more or other games in that setting. It's a numberless, diceless variant on the concept of the The Great Game that was used to create 2300AD's back-story and can be used much the same way.
I've only recently gotten my hands on a copy of Microscope and haven't gotten a group together yet to try it, but I am looking forward to it. Once I've gotten some actual play underway, I'll revisit it here and post some more.