A very nice graphic is making the rounds (though I believe it originated in a 2007 issue of Time Magazine) which shows Manhattan's population density by day and by night. The difference is striking:
Happily, the density bars mimic the placement of Manhattan's skyscrapers - this follows because obviously the tallest buildings support the highest population density. What's striking, however, is the breakdown - commercial buildings are indicated in the "day" graph and residential buildings in the "night" graph.
This means the building categories can be identified merely by time rather than any other dimension. Ordinarily, asking someone to pick out whether a building was primarily commercial or residential would probably involve a study of tenants or usage. Here we see that merely counting the daily/nightly inhabitants suffices: a clever use of data to avoid an otherwise grueling task.
Aside, the density heights have clearly been scaled for dramatic purposes. This means the two panes are not intuitively comparable; for example the daytime population seems much higher than the nighttime. While there's certainly more people working in the city than living in it, the exponential scaling makes it difficult to see just big a difference there is.
(via Kenny Herman)