National Geographics Xpeditions: Hundreds of printable maps

http://www.nationalgeographic.com/xpeditions/atlas/

World Factbook: hundreds of maps, up to date

http://www.odci.gov/cia/publications/factbook/docs/refmaps.html

Digital History: historic maps

http://www.digitalhistory.uh.edu/maps/maps.cfm

Library of Congress Map Collection

http://memory.loc.gov/ammem/gmdhtml/gmdhome.html