IPUMS USA : Integrated Public Use Microdata Series
http://www.usa.ipums.org/usa

IPUMS-USA consists of more than fifty high-precision samples of the American population drawn from fifteen federal censuses and from the American Community Surveys of 2000-2008. … IPUMS is not a collection of compiled statistics; it is composed of microdata. … researchers must use a statistical package to analyze the millions of records in the database. A data extraction system enables users to select only the samples and variables they require. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog. This has been added to Statistics Resources Subject Tracer™ Information Blog.