ICPSR[1]: The Inter-university Consortium for Political and Social Research maintains and provides access to thousands of social science datasets for research and instruction. This is a first place to start in looking for social science data for secondary analysis.
Roper[2]: The Roper Center maintains several thousand polls taken in some 70 foreign countries. Data can be downloaded by topic, by question, or by dataset.
United States Decennial Censuses:
The Government Information Department has a rich collection of official copies in print of the Census of the Population and Housing of United States conducted between 1790 and 1970.
We also have a series of GeoLytics Census Data Products for 1970, 1980, 1990, and 2000 Census;and CD/DVDs for 1990 and 2000 Census distributed by U.S. Census Bureau.
Resources for Data Analysis[5]: Recommended resources (online tutorials and books) for social science data analysis, complied by software packages and level of proficiency.