General Data Sources

www.Data.gov
www.USA.gov
The World Bank
CIA World Fact Book
NationMaster
data.world

Kaggle
UCI Machine Learning Repository

Census Data

U.S. Census Bureau: 2010 Census Data

Economic Data

International Monetary Fund
National Bureau of Economic Research
FreeLunch.com

Education Data

U.S. Department of Education: National Center for Education Statistics
Education Policy and Data Center

Election Data

American National Election Studies
Election Data Services

Energy Data

U.S. Energy Information Administration: Data Tools & Models
National Renewable Energy Laboratory

Environmental Data

National Climatic Data Center: National Environmental Satellite, Data, and Information Service
Quantitative Environmental Learning Project

Health Data

National Institutes of Health
Centers for Disease Control and Prevention

Polling Data

FiveThirtyEight
Gallup Polls

Social Media Data

Viralheat: Social media data

Sports Data

ESPN (requires data scraping)
FiveThirtyEight
NFL "Big Data Bowl"

Links to More Data Links

Social Data Sources
Data Information and Services Center
Data Surfing on the World Wide Web
Online Data Sources