Other useful links: 

U.S. Census 2000 TIGER/Line Data, ESRI (data in shapefile format)

Digital Chart of the World, Penn State University Libraries (data in Arc/Info export format)

IRI/LDEO Climate Data Library (by the International Research Institute for Climate Prediction)

[Seq] SEQUOIA 2000 Global Change Research Project, University of California, Berkeley.

[Tig] Tiger/Line files, Geography Division, U.S. Census Bureau.