Widely-Used Databases for Finding Data / Publicly-Available Data Sets
Inter-University Consortium for Political and Social Research (ICPSR)
General Social Survey (GSS) Cumulative Data File Archive
Centers for Disease Control and Prevention (CDC)
Social Explorer Census Data
Public Opinion Polls (Roper Center)
United States Agency for International Development (USAID)
Pew Research Center

Miscellaneous Data Sets
Book of the States
DW-Nominate Scores
State and Local Government Finances Database
CQ Press Voting & Elections Data
Newsbank's America's News Data
The ARDA Religious Denominations by County
Institute for Research & Education on Human Rights
Ballotpedia
The Dynamics of Collective Action
Racial Segregation Indices
2008 Elections Information
Open States
NCSS (NTEE) IRS Nonprofit Data
1990, 2000 Income Inequality (Gini Coefficient) Data for U.S. Counties (U.S. Census Bureau)
Alternate: 1970-1990 Income Inequality (Gini Coefficient) Data for U.S. Counties
Alternate: 2000 Income Inequality (Gini Coefficient) Data for U.S. Counties
Behavioral Risk Factor Surveillance System
CDC List of Marijuana Use Data

General Cleaning/Merging Data
List of 50 States
National Association of Counties