In fact, I googled "City Generator", but I don't know what you meant by it. It gives lots of links to fantasy "city populators" which spit out numbers of trolls and such.

SketchUp is a good tip, though it's more like building a scale model of a city than generating one. Unless there is a script similar to RPG City creator, to generate the "ordinary" houses. I like the idea of 3D but don't have the time to plot 1000s of buildings. Also, 3D demands a greater degree of realism, while a map can be more abstract.