For researchers that need the detailed decennial U.S. Census data tables and American Community Survey sampled data tables normally available in American FactFinder, check out the National Historical Geographic Information System (NHGIS). NHGIS provides free aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2011. Knowledge of GIS is not required; data tables are downloaded separately from the GIS shapefiles.
Registration is required, but the data and GIS shapefiles are free. The interface will be familiar to frequent users of U.S. Census data.