Nationwide:

http://www.nationalatlas.gov/

• National Atlas.  Includes numerous data available for the United States.  Includes premade maps as well as various data for map making in the categories of Agriculture, Environment, Map Reference, Biology, Geology, People, Boundaries, Government, Transportation, Climate, History and Water.

http://www.nws.noaa.gov/geodata/

• National Weather Service GIS - AWIPS Shapefile Database. Features data on states, provinces and counties, cities and urban areas.  Also includes meteorological and climatological data.

http://nationalmap.gov/viewer.html

• Use The National Map Viewer and Download Platform to visualize, inspect, and download the most current topographic base map data and products for free.  Data include: Elevation, Orthoimagery, Hydrography, Geographic Names, Boundaries, Transportation, Structures, and Land Cover, while products include: US Topo and Historical Topo Maps. The National Map Viewer also allows visualization and identification queries (but not downloads) of Other Featured Data, to include Ecosystems, Protected Areas, Gap Analysis Program Land Cover, Hazards, Weather, Wetlands, Public Land Survey System, and National Park Service Boundaries.

http://nationalmap.gov/historical/index.html

• Historical Topographic Maps.  The processing of this invaluable collection is well underway, with more than 141,000 GeoPDF maps of the approximate total 180,000 currently available for download. Over the next months the number of files available, in a variety of published scales, will continue to expand until all of the topographic maps created throughout the Survey's history are once again available to the public.

http://www.mrlc.gov/finddata.php

• Multi-Resolution Land Characterization (MRLC) consortium.  Use this to find land use and land cover data to the U.S.  Utilizes remotely sensed imagery and raster based data.

State data:

http://www.cyberdriveillinois.com/departments/library/maps/maplinks.html

• Illinois State Library of recommended map links.

http://www.isgs.uiuc.edu/nsdihome/

• Illinois Natural Resources Geospatial Data Clearinghouse.  Includes data for Illinois with digital orthophoto up to 2006, individual topographic data for Illinois and statewide data on Geology, Elevation, and Infrastructure.

http://msdis.missouri.edu/data/lidar/index.html

• Missouri Spatial Data Information Service.  Includes basically the same data at the Illinois Natural Resources Geospatial Data Clearinghouse.