The David Rumsey Collection: Online historic maps beautifully scanned and imaged. 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.
Topo Maps Online
UC & Stanford Map Collections
British Library Map Collections
Colorado School of Mines
Harvard Map Collections
New York Public Library
Penn State Maps Library
University of Connecticut MAGIC Map and Geographic Information Center.
University of Oregon Map Library. Many useful links.