As part of a recent data workshop I was running in New York, one of the datasets we used during the course of the day was this one from citymayors.com listing population density in various world cities.
For one of the examples I began to play with the idea of displaying a cities density (people per square kilometre) by creating city-like typographic expressions of that density. The more crammed and taller the buildings within the boundaries of the type, the more dense a place. I think you really feel a sense of claustrophobia when you view Mumbai, especially when you compare it to somewhere like Portland or Houston. I also like that the city becomes a bit harder to read the more dense it is, suggesting a real lack of breathing space.