Archive of data resources from research in the social and behavioral sciences, including 21 specialized collections of data in education, aging, criminal justice, substance abuse, terrorism, and other fields.
The U.S. government's official portal for open data. Provides access to data from numerous federal agencies in open, machine-readable formats, as well as tools and resources to help utilize them.
A new tool from Google that searches multiple data sources. However, it does include a large number of results where one has to pay for data, so use with care.