ARTstor - Artstor's images include historic maps. Limit to "Geography" in the browse classification and then type in "map."
David Rumsey Maps - Part of the Insight Collection, spectacular, large-scale images of historic maps
Historic Map Page - Duke Cartography librarian Mark Thomas' page of world-wide sites of historical cartography.
World-Wide Web Virtual Maps - one of the oldest (and perhaps the most amateurishly laid out!) collections of virtual maps. Still an excellent source