Originally developed as part of the Center for Technology in Government's GIS Cooperative Project, the Metadata Repository was created to provide one central location where State agencies and local governments can list the GIS data sets that they have. GIS users can browse the list, or search for specific keywords, to find out what data is available, who holds it, and how it can be obtained. In some cases, users will be able to go one step further, and download the actual data set.
The Geography Network is a global network of geographic information users and providers. It provides the infrastructure needed to support the sharing of geographic information among data providers, service providers, and users around the world.
Federal Government Geographic Data
A frequent question is how to find administrative boundaries for foreign countries in order to do thematic mapping. Here's a very useful site.
The National Historical Geographic Information System (NHGIS) provides, free of charge, aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2010.
Not online (on CD's at Olin Reference), but a very accessible source for 1970 and 1980 boundary files.