DATA.gov -- Provides access to datasets generated by the Federal Government and includes descriptions of the datasets (metadata), information about how to access the datasets, and tools that leverage government datasets.
Data on the Net -- Links to websites offering numeric social science statistical data, data catalogs, data libraries, social science gateways, etc. Provided by University of California, San Diego.
Harvard Dataverse Network -- The world's largest collection of social science research data. Free and open to all researchers worldwide to share, cite, reuse and archive research data. Registration required.
re3data.org -- Global registry of research data repositories that covers research data repositories from different academic disciplines.
UKDataService -- Provides access to over 6,000 digital data collections for research and teaching purposes. Covers an extensive range of key economic and social data.
NCDC Data Services -- The world’s largest provider of weather and climate data. Land-based, marine, model, radar, weather balloon, satellite, and paleoclimatic are just a few of the types of datasets available.
SEDAC -- Socioeconomic Data and Applications Center, is one of the Distributed Active Archive Centers of the U.S. National Aeronautics and Space Administration. Focuses on human interactions in the environment.