When one thinks of the greatest city in the history of pulp fantasy, one immediately thinks of the City of the Black Toga, Lankhmar. And when one thinks of the greatest city in the history of fantasy roleplaying, one immediately thinks of Judges Guild's City State of the Invincible Overlord. First published in 1977, CSIO made a huge splash at the time of its release and it remains, in my opinion, the gold standard for urban sourcebooks over 30 years later.
One reason for that was its huge 34" x 44" map (in four sections), detailing over 300 individual locations, along the NPCs who inhabited them. This opened urban adventuring to sandbox-style play with great ease. Players could send their characters to wander aimlessly through the City State, checking out its various shops and sights without the referee having to worry about the daunting task of creating it all himself. Of course, in typical Judges Guild fashion, these details were sparse and open-ended. This let each referee tailor the City State to his own campaign, making this product very much a "playing aid," just as it bills itself.
CSIO really was a very flexible and easy to use product. I have filed off the serial numbers and re-tooled it for use in many campaigns, most memorably my Greyhawk campaign of old, where it stood in for the City of Greyhawk itself (there being no official alternative at the time). The City State is also the basis for Adamas in my current Dwimmermount campaign. Oddly, I've never actually used it in a genuine Wilderlands campaign. My few Wilderlands campaigns were all set far away from the City State, which makes me a bit unusual, I suspect.
I don't think it's an understatement to say that Judges Guild played a bigger part in establishing most of what I consider the essential elements of old school play than any company outside of TSR. Beyond that I'd say that JG did even more than TSR when it came to developing certain elements, most notably wilderness/sandbox and urban play. CSIO shaped many people's perceptions of what a D&D city ought to be and, as I said, I've yet to see a more immediately useful urban environment for gaming than this one. Like the Wilderlands of which it's a part, the City State is a terrific pulp fantasy goulash that borrows gleefully from many sources of inspiration, creating a city that's not only a great place to adventure in its own right but one that feels as like a D&D city should -- boisterous, somewhat incoherent, but brimming with possibilities.
You can still find copies of this great game aid on eBay and from used game sellers, but they're often very pricey. Necromancer Games produced a 3e version of it several years ago and, like all their JG stuff, it's excellent. It really is a pity that they're not continuing to produce new Wilderlands material, but that leaves an opening for guys like James Mishler, so I can't really complain. In any event, City State of the Invincible Overlord remains one of my favorite RPG products of all time and, in terms of its influence, it certainly ranks highly indeed.