Manhattan Population Explorer:

The visualization you see here is a model of the dynamic population of Manhattan, block-by-block and hour-by-hour for a typical week in late Spring. The population estimates are the result of a combination of US Census data and a geographic dispersion of calculated net inflows and outflows from subway stations, normalized to match population daytime and nighttime estimates provided by a study from NYU Wagner.

What a fascinating look at the population changes of Manhattan throughout the day and week.