Data.gov https://data.gov
Open source data from U.S. Government sources
United States Census Bureau
The Leading source of high-quality data about the Nation's people and economy
Census Interactive
https://www.census.gov/library/visualizations/interactive.All.html
See especially the American Community Survey
https://www.census.gov/programs-surveys/acs/guidance/subjects.html
Statistical Abstract of the United States (ceased 2011)
http://www.census.gov/library/publications/time-series/statistical_abstracts.html
The authoritative and comprehensive summary of statistics on the social, political, and economic organization of the United States. Library also has 1937-2010 at C 3.134 in Government Documents
Climate
U.S. Global Change Research Program
https://www.globalchange.gov/browse/reports
Criminal Justice
Bureau of Justice Statistics http://www.bjs.gov/
Statistical information on crime, criminal offenders, victims of crime, and the operation of justice systems at all levels of government
Education
National Center for Education Statistics http://nces.ed.gov/
Data for public schools, public school systems; limited data on private schools by state, county, individual school or school district.
Energy
Energy Information Administration https://www.eia.gov/
Data, reports, forecasts, analytical content, and documentation; as tools for user-customized data queries.
Healthcare Costs and Insurance
U.S. Census Bureau https://www.census.gov/topics/health.html
From Current Population Surveys, American Community Survey, and Survey of Income and Program Participation
Agency for Healthcare Cost and Utilization [DHHS] http://www.ahrq.gov/research/data/index.html
National Health Care Disparities Report
https://www.ahrq.gov/research/findings/nhqrdr/index.html
Immigration (Department of Homeland Security)
See especially Yearbook of Immigration Statistics for Detailed Annual Tables
https://www.dhs.gov/immigration-statistics/yearbook/2017
Core reports on immigrants by legal status. Also quantitative studies and research on immigration.
Poverty, Income, Health Insurance
U.S. Census Bureau http://www.census.gov/topics/income-poverty/poverty.html
Public Welfare Programs
Social Security Research, Statistics, and Policy https://www.ssa.gov/policy/
TANF (Temporary Assistance for Needy Families) Office of Family Assistance
http://www.acf.hhs.gov/ofa/programs/tanf
USDA Food and Nutrition Service Programs (SNAP, WIC, etc.)
https://www.fns.usda.gov/research-analysis
Unemployment
Bureau of Labor Statistics http://www.bls.gov/bls/unemployment.htm