I've always been rather fond of the logos used by the 'city-ist' suite of sites i.e. Gothamist, Londonist, Shanghaiist etc. (I think the logos are by Sam Park, but do correct me if I'm wrong.) Here's a collection of them I made a month or so ago. It's interesting to see what buildings or urban features are used to denote the city, and also how well they work as visual identifiers. Together, they form a kit of parts that could be used to form alternate skylines, compiled cities.
(There's a version at Flickr you can annotate)
Reminds me of discussion started by Jon Bell a couple of years ago, in a Things magazine entry on buildings as logotypes, continued in the comments there).