Are you looking for United States census data and GIS-compatible boundary files? The National Historical Geographic Information System (NHGIS) provides a full compliment of aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2000, free of charge.

In addition, the NHGIS Data Finder (which requires free registration) provides access to aggregate census data via an easy-to-use web based interface.

The NHGIS also offers the Social Explorer, an online mapping interface that allows you to create and save maps, slideshows, and reports, all online. Data for Social Explorer are available beginning in 1940. Social Explorer is based at Queens College of the City University of New York.

The NHGIS is a project of the Minnesota Population Center at the University of Minnesota.

For more GIS data sources, check The Earth Sciences and Map Library's website which has an extensive list of links where you can find census and other GIS data available online.