"Data.gov provides descriptions of the Federal datasets (metadata), information about how to access the datasets, and tools that leverage government datasets."
ICPSR maintains and provides access to a vast archive of social science data for research and includes many large time-series datasets. International in scope.
Index to governmental and private statistical data.
Covers data from U.S. federal and state governments, universities, private associations and organizations, independent research groups, as well as international and intergovernmental organizations