Spatial Information Resources 

David Rumsey Historical Maps
Over 11,000 historical maps primarily from North and South America, most of them adjusted for use with the GIS browser embedded within the site.

Perry-Casteneda Library Map Collection
A list of online maps from one of the best collections in the world.

Library of Congress Map Collections 1500-2004
"The focus of Map Collections is Americana and Cartographic Treasures of the Library of Congress; the collection is organized according to seven major categories."

Historical USGS Topographic Maps
From Maptech, high-resolution historical topos for selected states.

ReliefWeb Map Centre From the United Nations Office for the Coordination of Humanitarian Affairs, a database of information-rich maps of international crises and conflicts.