I've also heard that Freeport and Bluffside are quite good however if you're willing to troll through 2nd hand products or get an ESD (I'm not sure if it is available on ESD or not though), you should try getting the World of Greyhawk boxed set which has in it the City of Greyhawk supplement (I think that's how it is anyway) as it's pretty darn cool, even if Gygax wrote it
Aside from that resource, there's plenty of other stuff for the City of Greyhawk around the traps if you're willing to search for it. There was a ripper map done up by one of the Greytalkers, in one of the first Living Greyhawk Journals which may or may not still be available for backorder from the RPGA. It's also available at Maldin's website, here (beware that the page requires java and is a big download):
http://www.timebomb.on.ca/denist/greyhawk/cogh.html
Greyhawk has to be one of THE most detailed cities in any game system. Aside from the reference I mentioned above, it has numerous other OOP products based on it so as long as you're up to the challenge of finding the OOP material, there is an absolute BUTTLOAD of stuff out there for it that should keep you going for a very long while.
Ugh, I keep forgetting stuff and having to come back and edit the post!
Also, you might want to check out Canonfire! below as it has a lot of free, fan-based and otherwise, material for the World of Greyhawk.