Direction Maps:

MapQuest - http://www.mapquest.com

Historical Maps:

Historical Atlas of the 20th Century - http://users.erols.com/mwhite28/20centry.htm 
Includes information on socio-economic trends, systems of government, wars and religion.

Panoramic Maps (1847-1929) -  http://lcweb2.loc.gov/ammem/pmhtml/panhome.html
Nonphotographic representations of cities portrayed as if viewed from above at an oblique angle, from the late 19th and early 20th centuries.

University of Georgia Hargrett Rare Maps Collection - http://www.libs.uga.edu/darchive/hargrett/maps/maps.html 
Rare maps from 1600-1870.

David Rumsey Collection - http://www.davidrumsey.com
Focuses on 18th and 19th century North and South American cartographic materials.

United States Maps:

United State Geological Survey - http://www.usgs.gov

National Atlas of the United States - http://www.nationalatlas.gov

Maps Collection Home Page - http://www.memory.loc.gov/ammen/gmdhtm/gmdhome.html 
A searchable collection of maps from the Library of Congress.

Earthshots: Satellite Images of Environmental Changes - http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontents

TopoZone - http://www.topozone.com

World Maps:

All the World Maps (EmbasssyWorld) - http://www.embassyworld.com/maps/maps.html 
Provides access to national maps and maps pinpointing major cities for countries.

Perry-Castaneda Maps library - http://www.lib.utexas.edu/maps/index.html 
Great site for maps.  Maps of current and general interest.

HRW World Maps - http://go.hrw.com/atlas/norm_htm/world.htm 
Continent/World Region maps

National Geographic Maps - http://plasma.nationalgeographic.com/mapmachine 
Access to political and physical maps and information for anywhere in the world by the National Geographic Society.

United Nations (Cartographic Section) - http://www.un.org/Depts/Cartographic/english/htmain.htm 
General country and special issues maps.