Commercial (subscription-based) sources
Statistics and data
Statista -- search by keyword or topic to find statistics; source details may provide pointers to underlying data.
Free online resources
U.S. Census: age, ethnicity, income, educational attainment, etc.
State and County QuickFacts - includes statistics for all states and counties, and for cities and towns with more than 5,000 people.
Census 2010 Interactive Population Map - details about population, age, race/ethnicity and housing status, for counties and subdivisions within counties.
American Community Survey - includes updates and projections since the decennial census.
Sample search -- Educational attainment in Trenton:
Explore Census Data - Includes examples for tables, data profiles (at the state, county, or city level), data visualizations and mapping, and more. Topics include population, housing, educational attainment, employment and more. For help, see the FAQs and other guidance at What Is Data.Census.gov?
EasiDemographics / this is mostly a fee-based service, but look for the link at the top of the page to free reports, ring studies, maps, and more -- "The Right Site for the Web Census 2010 Reports" [a brief registration form is required]
Sample search:
Quick Reports, Quick Tables, Site Analysis, Quick Maps and Profile Analysis are other useful analytical features included in The Right Site.
Multi-disciplinary social science data
ICPSR (the Inter-University Consortium for Political and Social Research)
[Note that the two databases listed below are configured to accommodate high-level consumer research tasks, and their user interfaces are not simple or intuitive.]
National demographic analysis and lifestyle trends
Experian: Simmons National Consumer Survey and National Hispanic Consumer Survey; also Simmons Connect (which measures digital behaviors on personal computers, smartphones, and tablets). [Select "Resources" from the menu at the top of the page for a user guide and other helpful information.]