Census of Population and Housing - Every 10 years since 1790
Census 2010 - American Factfinder.
University of Virginia Historical Census Browser - state and county data
Census compilations from 1890 and before are included in the U.S. Congressional Serial Set.
Beloit College Library -- 1910 and 1930-1980 census volumes are in the government documents collection, upper level, shelved under SuDoc number C3. Agriculture and manufactures census volumes are also in government documents, upper level, SuDoc C3.