Suggested Free Sources

Worldwide elevation

http://geoengine.nima.mil/muse-cgi-bin/rast_roam.cgi

Worldwide map of countries

http://en.wikipedia.org/wiki/Global_Administrative_Unit_Layers_(GAUL)

http://www.bluemarblegeo.com/products/worldmapdata.php?op=download

(a lower resolution map)

US time zones

http://www.nationalatlas.gov/mld/timeznp.html

US states

http://www.nationalatlas.gov/mld/statesp.html

US counties

http://www.nationalatlas.gov/mld/countyp.html

http://www.census.gov/geo/www/cob/st2000.html#shp

US ZIP codes

http://www.census.gov/geo/www/cob/z32000.html for 3 digits

http://www.census.gov/geo/www/cob/z52000.html for 5 digits

Worldwide

http://www.unearthedoutdoors.net/global_data/true_marble/download

Worldwide satellite map from NASA

http://earthobservatory.nasa.gov/Features/BlueMarble/

with a download mirror at

http://mirrors.arsc.edu/nasa/world_500m/

http://neo.sci.gsfc.nasa.gov/Search.html

(a lower resolution map)

USA

http://www.nauticalcharts.noaa.gov/mcd/enc/download_agreement.htm

Worldwide

http://geoengine.nga.mil/geospatial/SW_TOOLS/NIMAMUSE/webinter/vmap0_legend.html