University of Chicago Library has choropleth maps of selected 1990 Census variables, primarily of Chicago and near suburbs by Census Tract.
The Urban Information Center at the University of Missouri St. Louis maintains an extensive collection of public data files.
CIESIN (Center for International Earth Science Information Network) at Columbia University has an extensive site featuring DDCarto which provides digital coverages of Census demographics, and DDViewer, a query tool for Census data. They also have a Demography page with a variety of demographic information.