|Microscope by Ben Robbins from Lame Mage Productions|
In a nutshell - Microscope is world building as a role playing game. I'm planning on introducing it to my group the next time we need to create a setting for a sandbox campaign. Which, unfortunately, is going to be awhile since I'm currently running Star Trek Adventures with a couple of games featuring well-established settings planned after that.
Microscope has no gamemaster. Instead, players take turns presenting ideas for historical events within the setting that the group has come together to build. Each player has a free hand to move up and down the timeline. The detail level begins with an elevated view ("there was a war at this location between these parties that falls within those dates"), can focus all the way down into scenes between individuals during these events ("this is the meeting between the folks who planned out the final battle of that war"), and can be dialed in and out as the players desire. The scenes are where the roleplaying comes in - the players take on the roles of characters within the scene in question. Discussing ideas with other players only takes place within the context of playing out those ideas during roleplaying scenes. This creates a dynamic where players collaborate by building on what other players create, but don't interact with each other's ideas through conversation. This is intended to prevent any one player from dominating the creative process. Players can't shoot down ideas before they can be fully explained and explored. Having a mechanic to make sure that every player gets their turn is important for a game without a gamemaster.
Using Microscope during the formative period to create a roleplaying campaign setting avoids the dreaded infodump. Since everyone - players and gamemaster included - had an active role in making the setting, everyone involved is already familiar with the aspects of that setting. This avoids the problem of having to stop play to present background to players who are not already invested in the world that the campaign takes place in.
Overall, I believe Microscope to be a solid approach to collaborative world-building. It places everyone on a level playing field and encourages active participation. The mechanic of playing out scenes during key events helps to invest players in the setting. It is unfortunate that I won't have the chance to use it anytime soon.