(Cross-posted from the other thread about this map.)
That map is from the late 3.5-era Expedition to the Ruins of Greyhawk superadventure that I wrote with James Jacobs and Jason Bulmahn. I specifically wrote the parts that take place in the city itself, and I made sure that the map was based on the excellent poster map Denis Tetreault did of the City of Greyhawk when I was editor of the Living Greyhawk Journal, and which appeared in LGJ #2. These issues sometimes come up on eBay, but they can get pretty expensive.
Denis's map is essentially the map on the WotC site (which I think was done by Mike Schley), but more in the style of the old Harn maps, in that the buildings are stark white shapes.
If you really want a huge resolution version of the map, the LGJ poster can't be beat.