Ptolus: City by the Spire
My first Dungeons & Dragons campaign was a series of adventures contained in Dungeon Magazine which collectively came to be known as the Shackled City. It was a page-opener for me in many ways; in addition to being the first campaign dubbed an "adventure path" (Dragonlance preceded it by 18 years in concept), it was also the first one I ran to completion and the first stable group of gamers who stuck with me through high school and well into college. Over a decade's worth of fun memories.
As for the Shackled City itself, it centered around the aptly-named settlement of Cauldron, built within the inner ring of a dormant volcano home to a large central lake in the middle of a jungle. A foul cult dwelling within the halls of power and darkest depths alike sought to bring the city to ruin, and many of the adventures were connected in the growing awareness and eventual stopping of their plot.
Using a central area for a whole campaign was a clever one, as it allowed the GM to reuse familiar locations and NPCs to give a better connection to the area. While most adventure paths sought to replicate this feel, the often nomadic nature of most campaigns meant that players would venture from location to location to complete a task, only to pack up and head off to the next place. Dragonlance tried to tie in themes with recurring NPCs and a sense of history, but overall I found in my years of play that setting a campaign around a city leant itself to some of my best gaming sessions.
For that reason, I decided to compile a list of city-centric sourcebooks for the Dungeons & Dragons and Pathfinder role-playing games. I'm only including sourcebooks which focus largely on livable locations; megadungeons of ruined cities do not count, nor do adventures which take place in said cities but are more or less restricted to the confines of a single plot. If any of my fine readers have suggestions, or if I happened to miss a particular interesting metropolis, please let me know!
Lesserton & Mor (no longer on OneBookShelf)
Shackled City Adventure Path
Sharn, City of Towers
Shelzar, City of Sins
Sheoloth, City of the Drow
City of Stormreach
Vornheim, the Complete City Kit
City of Splendors, Waterdeep
World's Largest City