Maybe I'm a huge geek, but I find this kinda shit fascinating:
Largest cities in the United States by population by decade
From Wikipedia, the free encyclopedia
This entry tracks and ranks the population of the largest cities in the United States by decade, starting with the 1790 Census. For 1790 through 1990, tables are taken from "Population of the 100 Largest Cities and Other Urban Places in the United States: 1790 to 1990." For year 2000 rankings, data from the Census Bureau's tally of "Cities with 100,000 or More Population Ranked by Selected Subject" is used.For further research on year 2000 urban population, the County and City Data Book might be instructive.
A few examples (for the full list click here):