There are, of course, the city supplements for various worlds: Ptolus, Freeport, City Stat of the IO, Waterdeep, Sharn, etc. However, even if you are not running a Freeport campaign, I found Denizens of Freeport to be a useful supplement. Most class books related to rogues, bards, and assassins also have city info in them to various degree. Looking for back issues of Dungeon with urban adventures in them can also be a good resource.