For easy reference, here's a collection of some worldbuilding threads. If there are any missing, let me know and I'll add to it.
Question Game Threads
[Banestorm] Fleshing out the Djinn Lands Made some new detail for the Djinn Lands of Yrth and one of the djinn there.
[GAME] Orcish Ag Bash! Queen orcs and spirit circles for an eel-based farming system.
[GAME] Surveil a Superspy Setting! Directorate S, a private superspy agency backed by the world's third richest person, protects the world from a range of threats.
[GAME] Magic Academy/University, World Building An elite magic school founded in Quebec but located on the opposite coastline.
[GAME] After the End, Worldbuilding An environmental-nuclear apocalypse leaves uplifts and hybrid humans on the rise.
[GAME] Assemble a Space Station! A space station built inside an abandoned precursor base, and some details about the politics of the setting.
Campaign Jam: Magic Vrs. Technology A world with opposing camps using exclusively either magic or technology.
[GAME] Conceive a Cross Dimensional Fantasy Milieu - Mercantile adventure in a multiverse containing worlds in crocodile skulls, tetrahedrons and a distorted in-between.
[GAME] Incorporate a Cyberpunk Megacorp - Focuses on a South American 2nd tier megacorp
[GAME] Collaborative World Building Dwarven city as a start - Based on some agricultural modelling for a dwarven stronghold
[Game] Animate a Monster Hunters Setting - 1900's World-spanning Occult Adventure
[Game] Generate a military sci-fi setting - A space ark brings dozens of alien races to an overcrowded solar system just as Earth becomes uninhabitable
[Game] Collaborative world building for DFRPG - A Dungeon Fantasy town to stock up on rumours, magic items and guild intrigue between adventures
Create a setting -- diesel punk awesomeness! - 1930s in a world dominated by Greco-Buddhist geopolitics
Shared space setting - Building the wormhole superhighway to the stars
[Game] Work Up a Steampunk Setting - Skyship villains and bathysuit soldiers fighting undersea wars
Question Game: Post Agricultural Fantasy Setting
[Game] Let's Make a Post-Apocalyptic Setting
[Game] Generate a Space Trader Setting - Aliens, birthright humans and divergers with plasma-tube jump drives spread across the galaxy
Setting Generating Game (Feb 2015) (Fantasy setting) - Dogfleet Pirates from the Naga Isles conquer the coastal City States with the aid of numina spirits
Setting Generating Game (Supers setting- Odin-1)
Setting Generator game 2: Odin2 (Supers)
New Year/New Setting: GURPS world design 'game' (Fantasy setting- Twilight World): This was more of a "reference chain" game- refer to something previously mentioned then add to it.
Also, some vote-based threads:
Vote Up a Setting Threads
Vote up a Fantastic (as in Fantasy) Setting
Vote up a Fantasy Setting
Vote up a SUPERS! setting
Vote up an Urban Fantasy Setting
Vote up a Space Opera
And for straight-up collaborative world-building:
Other Collaborative World Threads
Collaborative DFRPG Mega-Dungeon - A dungeon thrown together by the God of the Underworld, which includes a range of random but creative ways to challenge a party of delvers.