The ACS has replaced the old "long-form" version of the Decennial Census and is now used to collect the more detailed socio-economic information that the Decennial Census no longer gathers. Note that the ACS consists of multiple "data products" gathered over different ranges of time and covering different levels of geography. See https://www.census.gov/programs-surveys/acs/data.html for an overview of different ways to access ACS data.
American FactFinder is an extensive source for census statistics from the U.S. Census Bureau. Users can create data tables from the Decennial Census (2000 and 2010), the American Community Survey (2000-present), and the Economic Census (1997, 2002, and 2007) and download those tables into spreadsheet files. Note: American FactFinder has been decommissioned and is no longer available. Data are now available at: data.census.gov.
Carolina Population Center -- The CPC is an organization located at the University of North Carolina, Chapel Hill, that promotes demographic research. There are links to the research projects underway as well as a set of links to various demographic research sources.
CDC Wonder includes a site that provides annual population estimates for different age, race, and gender groups. Users can get estimates at the national, state, or county levels, from 1990 onwards.
The Census Bureau gathers a vast amount of social, demographic, and economic information about the United States through various means. The Subject Index is a good starting point for browsing and searching through the Bureau's wealth of information.
The Census Population Estimates site provides annual population estimates for different categories of age, race, and gender, extending back to 1970. Estimates are at the national and state levels, with some additional estimates for counties and metro areas.
The Current Population Survey is a joint project between the Bureau of Labor Statistics and the U.S. Census Bureau. The CPS is a monthly survey that collects basic socio-demographic information, labor force characteristics, and economic status. To access CPS data files, users can download the data from an FTP site or use various tools for creating tables from the microdata. Alternately, they can go to the National Bureau of Economic Research's CPS site or to IPUMS-CPS.
The ICPSR has an extensive collection of both aggregate data and microdata from the Census Bureau, including data from the Decennial Census, the American Community Survey, the Current Population Survey, the American Housing Survey, and other data collections. The holdings include much historic Census data, various one-off data collections, and TIGER boundary files.
As part of the Census Bureau's International Programs International Data Base (IDB) contains statistical tables of annual demographic data for 227 countries and areas of the world, covering topics such as population distributes by age and sex, components of population growth, morality, and life expectancy.
Oxford University's MigrationOxford research hub includes the International Migration Institute Network, which hosts various datasets on migration flows (total and bilateral) and migration policies for dozens of countries since 1945.
The IPUMS project at the University of Minnesota is an excellent source for Census data in the form of microdata samples from each decennial Census from 1850-2000 and from the American Community Survey for 2001 and onwards. See https://usa.ipums.org/usa/sampdesc.shtml for a list of the samples available via IPUMS. The data include standardized/harmonized variables for topics such as industry, occupation, race and ethnicity, and educational attainment, for easier comparisons of data over time. The microdata come with geographic tools and boundary files, to assist with geographic analyses of the individual-level data. IPUMS-USA is one of many IPUMS efforts - see https://www.ipums.org/ for the full list.
The IPUMS International project at the University of Minnesota provides access to microdata samples from population censuses in dozens of countries from different regions and income categories and includes both the original data and harmonized variables for easier comparison over time and/or across countries. See https://international.ipums.org/international-action/sample_details for a list of the available samples.
The World Bank provides annual updates on migration data and flows of remittances.
The MCDC provides various tools to extract data from the Decennial Census for 1980 onwards and for the American Community Survey, as well as for other federal data on topics such as unemployment, health insurance, poverty, and population estimates. The MCDC also has an extensive set of data and tools for working with geographies used by the Census Bureau.
NHGIS at the University of Minnesota distributes boundary files for Census tracts back to 1910 (varying by state), as well as historical and current aggregate Census data at various geographic levels. In addition to data taken from Census reports, NHGIS also distributes data on topics such as mortality and vital statistics, Depression-era bank closures, land cover and and climate, and historic Census data standardized into 2010-vintage geographies for time-series analyses.
OPR at Princeton University archives a number of datasets of interest to those studying demography and demographics, with a particular focus on migrants and immigration.
The Population Studies Center is a great site for interdisciplinary research on population studies. The site also maintains a significant collection of data.
National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program distributes annual county-level population data broken down by combinations of age, gender, and race/ethnicity, for 1969 onwards.
The S4 team at Brown University hosts various projects that make use of census data and geographic data to examine topics such as segregation, depopulation, urbanization, and tools to standardize historic U.S. Census data into current geographic boundaries. See https://www.brown.edu/academics/spatial-structures-in-social-sciences/projects for the full list of U.S. and international projects.
The Statistical Abstract contained a wealth of information on numerous socio-economic and demographic indicators for the United States. Historical data tables taken from the Statistical Abstract are available via the Internet Archive. While the Census Bureau has discontinued publication of the Statistical Abstract, the Abstract is now published by ProQuest and can be accessed in the A-Z University Libraries List. ProQuest provides a research quide on this product.
U.S. Citizenship and Immigration Services (formerly the INS) publishes the Yearbook of Immigration Statistics and other collections of statistical information on topics such as immigration flows, immigration enforcement, and estimates of unauthorized immigration populations.
This database is produced by the United Nations' Population Division and contains cross-national data on basic population demographics and vital statistics (e.g. birth and death rates, maternal mortality, population by age and gender and urban/rural areas, median age of population). See http://www.un.org/en/development/desa/population/publications/dataset/index.shtml for other data from the Population Division, cover topics such as migration, fertility, marital status, and population policies.