Air University Library Map Room
Buy maps, mapping software; GPS; Street Atlas USA; Topo USA
Buy maps, both paper and digital. World's Largest Map Store.
University of Florida Maps & Imagery Library
Perry-Castañeda Library Map Collection, University of Texas
One of the best collections of on-line maps that I've seen. Maps of all kinds from all over the world.
A service of Switchboard.com
Topographic maps. This site has got every USGS topographic map, orthophotomap, and aerial photograph in the entire United States.