The National Historical Geographic Information System (NHGIS) provides population, housing, agricultural, and economic data, along with GIS-compatible boundary files, for geographic units in the United States from 1790 to the present.
Baylor University belongs to the ICPSR Consortium which maintains a data archive of over 500,000 research files in the social sciences. It's specialties include: education, aging, criminal justice, substance abuse, and terrorism.