Where else besides universities – and the occasional government agency – can you find staff who are able to maintain *current* and comprehensive lists of data sources? Not too many places. Here are some well-regarded ones, listed alphabetically.
MIT Libraries, Find (GIS) Data
North Carolina State University, Geospatial Data Services
Smith College, Finding Data for Your GIS. Nothing extraordinary here about the resources, but good basic instructions on how to get students to do their own searches.
Stanford Library, Stanford GIS Center, Find (GIS) Data
Tufts University, Online GIS Data Sources
University of Texas @ Arlington, Spatial and Numeric Data