I tripped over this law on one of the NYTimes blogs and found it fascinating.

As stated in the blog,

The mathematics of cities was launched in 1949 when George Zipf, a linguist working at Harvard, reported a striking regularity in the size distribution of cities. He noticed that if you tabulate the biggest cities in a given country and rank them according to their populations, the largest city is always about twice as big as the second largest, and three times as big as the third largest, and so on. In other words, the population of a city is, to a good approximation, inversely proportional to its rank. Why this should be true, no one knows.
Zipf originally applied the law to the frequency of words in a text, but it applies to a wide variety of data sets with no good explanation.