Judd Karlman had an awesome idea about how to create a D&D setting that attempts to achieve the same kind of polyglot fantasy metropolis weirdness and anthropological complexity of China Miéville's New Crobuzon. Naturally, being a giant pervert for setting creation, I had to give it a try.
Located on a lush jungle island well-situated near several major trade lanes, the Maw is a vertical city built into the igneous walls of a dead volcano's throat, crossing it in places with spidery bridges. The city extends so deeply that most of it rarely sees sunlight. Its positioning makes it a perfect center of trade between the civilizations of the surface world, and those of the labyrinthine lands beneath. The primary citizen races are humans (ever the most ambitious of the surface races) and dwarves (the subterranean race humans are most comfortable dealing with).
Goblins - Most of the local archipelago is controlled by a small empire of jungle goblins. They are confident, organized, and as hairless and brightly-colored as poisonous frogs. All three goblin subraces live and work together with no acknowledged distinctions. The Maw is a completely separate political entity from the goblin empire--the Strand of Jewels--but a large number of goblins, hobgoblins, and bugbears have made their home in the city, attracted by the foreign wealth it offers, and eager to wield whatever political and business connections they might have throughout the archipelago to carve themselves a piece of it. For the most part, they've set themselves up as middlemen and facilitators in surface-underworld trade, storing good, changing money, and arranging transport from the island's teeming ports. They also have a substantial stake in local organized crime.
Umber hulks - A handful of quasi-civilized umber hulks--the descendants of slaves purchased from the drow and freed upon the death of their owner--live and work in the depths of the Maw. They're only barely sapient, incapable of vocal communication, stunningly dangerous, and subject to a number of repressive ordinances, but their tunneling ability puts even the dwarves to shame, and they've proven to be extremely useful members of the community. They're paid poorly, but as they do a tremendous amount of work and hardly ever seem to actually spend money, it's assumed that they must have a tremendous hoard of it, somewhere. Just what--if anything--they plan to do with it is one of the Maw's most widely speculated-upon mysteries.
Homunculi - The combination of human arcane experimentation, strange materials from the underworld, and the wealth brought by extensive trade has given rise to generation after generation of increasingly advanced alchemical constructs. The humans purpose-grow them for a wide variety of jobs, from message-carrying and guard duty to bookkeeping and research assistance. While even the most intelligent homunculus cannot legally be a citizen of the Maw--in fact, they technically have fewer rights than the umber hulks--the smarter and more humanoid varieties tend to be considered people by most locals, and are almost universally trusted as honest and incorruptible despite the fact that most technically have free will. As homunculi take on ever-greater roles within the production of more homunculi, it's possible they could become a race in their own right. Some would say they're already there.
Aboleths - Humans, dwarves, goblins make up the Maw's ruling council, but it's something of an open secret that a group of aboleths are pulling the strings. Just how much of their control is obtained through psychic domination and how much is simply the result of their having some very interesting artifacts to trade is uncertain. But if they have any kind of sinister agenda in mind for the city, it's apparently too slow and long-term for even the dwarves to notice. It's very possible that the whole Maw settlement was their idea in the first place, though, and it's apparent that they've got exactly the right blend of superhuman intelligence and utter ruthlessness to make a city like this work.
Kruthiks - Several decades back, an incautious tunnel expansion in the depths of the Maw disturbed a massive kruthik hive. The hive was destroyed at great cost to the city's defenders, but more than half of the kruthiks escaped. Ever since, kruthik incursions and infestations have been a regular hazard in the Maw, particularly in the deeper regions. Affected neighborhoods have taken to hanging up kruthik corpses--preferably fresh ones--to ward off attacks. This has given rise to the erroneous belief that even a small part of a kruthik--say, a hatchling's mandible--made into a pendant will protect its wearer from them. In truth, such a small amount of kruthik deathscent is actually much more likely to attract their attention.
The eidolon - In all of the Maw, the only organized resistance to the aboleths' rule comes from a cult of dwarves and humans who worship a rogue eidolon of tremendous power. This titanic, quasi-divine construct was unearthed in one of the Maw's farthest-reaching tunnel systems, and immediately seized control of the dwarven clan who discovered it. The eidolon is powerful enough to grant divine power to its followers, and to roam through the solid earth at will, avoiding detection by the city's authorities. The cult operates semi-secretly, with most of its members hiding their faith, but a few evangelists work openly to bring converts, in spite of the cult's outlaw status.