The David Rumsey Collection: Online historic maps with a map-based search. Local and worldwide coverage.
Perry-Castañeda Library Map Collection at the University of Texas. A great source for basic political and physical maps.
On-line Digital Geologic Map Database of the San Francisco Bay Region. Links to online USGS geologic maps.
Online Maps Collections at the Library of Congress.
USGS Topographic Maps
*Access limited to UCSC faculty, students, staff