American Community Survey -- Provides estimates of demographic, housing, social, and economic characteristics every year for all states, as well as for all cities, counties, metropolitan areas, and population groups of 65,000 people or more.
American Factfinder -- Interactive, searchable database to find population, housing, industry and business statistics from the U.S. Census. Data available from 2000-present. American Factfinder allows the user to compile census data into tables, maps and downloadable files, which can be viewed or printed.
DataFerrett -- Allows the creation of custom statistics from several datasets.
Census FTP Server -- The Census Bureau makes much of its raw datafiles available via an FTP server. This is for experienced data users familar with statistical software packages such as SAS, SPSS, Stata. Codebooks are provided for each dataset.
iPums.org -- Provides access to Public Use Microdata Samples (PUMS) which are Census datasets of geographic areas over 100,000 people.
NHGIS -- National Historic GIS. Provides access to GIS shapefiles and census data.
Current Population Surveys -- Joint effort between the Census and Bureau of Labor Statistics. Provides detailed labor force statistics of the U.S. population.