Online Data Catalog Sites

Stanford Libraries and Academic Information Services

http://www-sul.stanford.edu/depts/gis/web.html - very well-organized site with some special topic areas as well as the main clearinghouses. Check out the site's web bookmark file for even more sites.

Tufts GIS Data Resources Guide

https://wikis.uit.tufts.edu/confluence/x/k4BL- Tufts GIS staff maintain a listing of GIS data sites concentrating on international and US national clearinghouses, as well as New England states, cities and towns.

Harvard University - GIS Data Resource Pages

Harvard Geospatial Library - online geodata repository with map and keyword search through FGDC metadata.

GIS Data Sources - An extensive list of GIS data sources maintained and updated by the Harvard College Library.

More GIS Data Sources - An extensive list of GIS data sources maintained and updated by the Center for Geographic Analysis. 

University of Arkansas Libraries - Geospatial and Attribute Data Links

http://libinfo.uark.edu/gis/datalinks.asp - well organized data resource guide to state and national data.

US National Data Sets

For quick easy access to a multitude of nationwide data sets for the US, check the National Atlas (http://nationalatlas.gov/) - for data download, go to the link for "Mapping Professionals" and then "Raw Data".  If you are doing a national level GIS project or looking at a large region, this is a great site to use. If you want just parts of the US, use the National Map web site from which you can clip regions - http://nationalmap.gov/.

Free GIS Data by Region - Collins Software

http://www.collinssoftware.com/freegis_by_region.htm- this is an excellent  and extensive list of city, metropolitan, and state data sets, plus some good international data links, from Collins Software out of Houston, Tx.