Give a DM a chance and he'll pull out a powerpoint presentation explaining the cosmology of his home-brewed setting. That or he'll lament about his players having done something which has pushed his carefully produced story into the background as they instead concern themselves with some throw away detail.
This is the place where we can discuss settings and adventures for all the various RPG systems that there are. Made a new world for your new Pathfinder game? Struggling to come up with a suitable reason for your gang of werewolves to do some complicated werewolf thing for your Werewolf game? Need to know if there's any "canon" stuff about goblin deities in the default (and yet quite intentionally nebulous) "Points of Light"4e setting
To start us off, I'll include some bits and bobs about popular settings
World of Darkness
The setting used by Whitewolf for their collection of connected games involving vampires, werewolves, changelings, the people who hunt the aforementioned monsters and every so often, a brand new group of entities. This setting is the default for the WoD games, and for the most is something along the lines of the real world but with monsters that the overwhelming majority of people don't know about and/or choose to ignore.
The new edition of the rules streamlined the problems the setting had developed over the years (mainly that each of the player types had been considered as a stand alone game despite coexisting).
The newest, excluding the default 4e setting, for D&D. Developed during the last edition as part of a competition and favouring logical consequences of a world with magic to end up with a slightly pulp-y noirish setting. In this sense, it is a fair old departure from the usual D&D, which angers old men with beards.
For most people magical trains, living constructs as a player race and a poorly explained world which is made of three dragons on top of one another is a good time.
SAND! Set on a world called Athas, Dark Sun is another slightly off-beat but incredibly popular D&D setting. Ignored aside from some half-arsed articles in the magazines during the 3e days, this has been relaunched for 4e with some significant tweaks to make it a bit more playable.
If you like magic consuming the planet, sorcerer kings who might one day become dragons, half-dwarves, mantis-men and dying of thirst in the desert then this is the one for you.
The most famous and popular table top wargame also happens to have had several RPGs produced over the years. None of which have been all that popular. Despite this, it's a great setting, largely due to the sheer volume of fluff which is out there for it.
As a setting, it's largely concerned with the brutal lives of humans in a world where everything wants to eat them. For the most part you can think of it as a vaguely Germanic form of the Roman Empire set to a background of evil chaos gods, swarms of orcs and a race of ratmen who don't exist.
Pathfinder Campaign Setting
Originally, Pathfinder was just Paizo's improvements on D&D 3.5e, but as Wizards of the Coast moved on with 4th edition, Pathfinder picked up a fairly distinct flavour that was represented in the default setting - a world known as Golarion.
This setting is a bit of an oddity, as while lots of books and maps detailing specific cities or regions have been put out, the core book isn't set for release for another two months. Details are spread through the vast number of books, but this is purposely set out to be a fairly by the numbers fantasy setting along the lines of D&D Greyhawk's (albeit with the existence of a few guns here and there).
A post-apocalyptic world dreamt up in the late 70's. The world is now filled with mutants pouring over the remains of earth. Recently released using the 4th edition rules (although with some tweaks, as this is quite the divergence from your typical game of D&D), this is actually now in its seventh edition. Aimed at light, humorous and fast fun rather than supporting longer term play.
So, that's a pretty random smattering of campaign settings (with a primarily D&D focus, but I think that is more a product of how D&D hosts dozens of settings, something which is fairly atypical). I've not really said anything about adventures, but I may add a second post with some of the more famous modules (The Temple of Elemental Evil and whatnot).
Which was the worst of the hundreds of D&D settings cranked out over the years?
Do you mourn the loss of official support for the Forgotten Realms setting? Or are you happy that that collection of half-baked ideas and overpowered NPCs who should save the day long before the PCs even catch wind of a problem is gone?
Did anybody else ever play through the "Night Below" adventures back in the AD&D days?
Has a successful game ever been run based on a television IP?