Inter-University
Consortium for Political and Social Research
http://www.icpsr.umich.edu/
The Consortium
says it's "the world's largest archive of digital social
science data. We
acquire, preserve, and distribute original research data
and provide
training in its analysis." The data files are searchable and
browsable; each
item retrieved includes a description, scope of the
study, dates and
geographic region, methodology, and information on
access. There's a data
use tutorial on the website as well.