A pretty good Primer from the Harvard Design School
Very useful GIS resources from Rice University
A Motley Assortment of Governmental Data
The GLOBE Ditigal Elevation Model from NOAA
The North American Transportation Atlas
US Governmental Maps at GeoData.gov
US Census data portal, including basic geographic layers
The “WONDER” portal for CDC health statistics (for importing to GIS)
Other medical GIS data sources, from Stanford
Map layers from the US National Atlas
Data from the Canadian Census
Data from the Atlas of Canada
GIS data from the New York City Department of Planning
Each U.S. state also maintains its own GIS portal (see Rice).
Data from the British Ordnance Survey.
Basic GIS data for most countries of the world: Global Mapping