The year 2010 Census was the most recent assessment of the population of the United States. Here are some useful links to information derived from that Census:
Start here to see what data from the 2010 census is available now: http://2010.census.gov/2010census/
factfinder2.census.gov : This new version of the American FactFinder is a work in progress and will replace the original (Legacy American FactFinder) later this year.
The Mapping the U.S. Census project of the New York Times allows you to immediately learn the 2010 and 2000 census population and the 2010 racial and nationality composition of the county your cursor is pointing to.
CensusScope: Your Portal to Census 2010 Data is an easy-to-use tool for investigating U.S. demographic trends, brought to you by the Social Science Data Analysis Network (SSDAN)at the University of Michigan. With eye-catching graphics and exportable trend data, CensusScope is designed for both generalists and specialists