Chernel. The map of Chernel is loosely based upon the planned city of Chendl the capital of Furyondy first developed in Fate of Istus and partly ruined in From the Ashes. Chernel goes beyond its source material and is a homebrew work in its own right, a layered PDF map with nearly 400 unique features, though still easily adaptable for any Greyhawk campaign.
If you want a highly detailed map to base an urban campaign around (other than The City of Greyhawk for once) then perhaps Chernel can get you started!
p.s. go back and check out Chris' layered map of the Flanaess over at Dragonsfoot.com