The Inter-university Consortium for Political and Social Research maintains a data archive of more than 500,000 files of research in the social sciences. It hosts 16 specialized collections of data in education, aging, criminal justice, substance abuse, terrorism, and other fields.
Data.gov provides descriptions of US Federal datasets (metadata), information about how to access the datasets, and tools that leverage government datasets.
From the Census Department, The American Community Survey (ACS) is an ongoing survey that provides data every year -- giving communities the current information they need to plan investments and services.
The ARDA strives to democratize access to the best data on religion, now including American and international collections and developing features for educators, journalists, religious congregations, and researchers.
The Dataverse Network provides access to data collections curated by the Odum Institute as well as collections owned by other institutions and individual scholars.
The GSS contains a standard 'core' of demographic, behavioral, and attitudinal questions, plus topics of special interest. Many of the core questions have remained unchanged since 1972 to facilitate time-trend studies as well as replication of earlier findings.