PLSS shapefiles by state
http://www.geocommunicator.gov/blmMap/MapLSIS.jsp

GIS Data Depot
http://data.geocomm.com/catalog/US/sublist.html

Illinois Natural Resources Geospatial Data Clearinghouse
http://www.isgs.illinois.edu/nsdihome/webdocs/browse.html

National Atlas of the United States
http://www.nationalatlas.gov/

U.S. Maps and Data - geodata.gov
http://gos2.geodata.gov/wps/portal/gos

USGS Seamless Data Warehouse
http://seamless.usgs.gov/

USDA NRCS Geospatial Data Gateway
http://datagateway.nrcs.usda.gov/

US Census Bureau
http://www.census.gov/geo/www/tiger/index.html

FEMA Flood Maps
http://www.fema.gov/hazard/flood/index.shtm

Tennessee Spatial Data Server
http://www.tngis.org/data.html

OIR-GIS Services, State of Tennessee
http://tnmap.state.tn.us/wpc/

USACE Mobile District Spatial Data Branch
http://spatialdata.sam.usace.army.mil/organizations/GIS/Data.aspx

Tennessee Wildlife Resources Agency
http://www.state.tn.us/twra/gis/downloaddata.html

NEW LINK 11/03/2011
http://freegisdata.rtwilson.com/