Very Large List of Useful GIS Data Sources

The PDF from an ESRI educational class has many links to GIS data sources and viewer utilities.

To access the PDF, just download the zip archive. It looks like you're downloading some sort of official ESRI course, but it's just a one-file pdf document that explains how to access a ton of free GIS data:

http://edcommunity.esri.com/arclessons/lesson.cfm?id=305