Environmental GIS for entire USA
http://chppm-www.apgea.army.mil/gwswp/GWLinks/stategis.htm (Another Link list page like this)
http://libweb.uoregon.edu/map/map_section/map_Statedatasets.html (Another Link list page like this)
http://unr.edu/homepage/daved/gislinks.html (Another Link list page like this)
http://sedac.ciesin.org/ Download socioeconomic data (population, environmental and census data) from around the world.
World Atlas and World Maps - Maps and Geography of the World - Lot of data for the world. Maps, cencus data, ect around the world.
GIS Data Depot has a incredible amount of GIS data. Some of the data is no longer free, see note below. You will need the county which you can get from an atlas or Expedia.com and you will need the name of the map which you can get from USGS Store Search Page or Geographic Name Info System. Once you have this go to GIS Data Depot and then select the state then the county then "Digital Raster Graphics (DRG) - 24K" then the topo, then download. Go to where it says "Click here to download: xxxxx" and then right click with the mouse and do "Save Target As..." Note: You now have to pay for much of the data on gisdatadepot (mainly topographic maps and aerial photographs. As of May 7, 2002 the cost was $25 for 150 Megs of downloads or $110 for 1 Gig. Being most topos are 3 megs that means a topo would normally cost $0.50 each. Not too bad.
http://drgmaps.com - This place sells entire states on CD-ROM and the price is very very good. For example you can get all of Tennessee (805 maps) for $50.
http://www.mapmart.com/ - I haven't used this place but seems to have info for all of the USA and some international info.
DEMs for the entire USA
DLGs for entire USA
Satellite/Aerial Images
http://www.spaceimaging.com/ Has some nice photographs from space
NGS Survey Marker Accuracy - Gives you information on accuracy of bench markers. (the David Doyle here is a different person than me)
Tennessee Dept of Transportation Standard Survey Division Resources