Between decennial census', the U.S. Census Bureau releases annual estimates of population.  Recall that ESTIMATES review past years, while PROJECTIONS look into the future.  Estimates are created by using the April 1, 2000 Census as a base, with births and deaths added to account for natural increase.  Tax return information is used to estimate migration into / out of an area.

 

The attached spreadsheet and Adobe .PDF file were published by the U.S. Census Bureau's Population Estimates Division, and contains municipal and county estimates for July 1st of 2000, 2001, 2002, and 2003.  Questions should be directed to that site, which contains links to the actual estimate methodology and definitions, as well as downloadable files.