First up - Birmingham Alabama Public Library
http://bplonline.cdmhost.com/cdm/search/collection/p4017coll7!p15099coll3/order/date
1441 online maps dating to before Columbus.
Next - The Barry Lawrence Ruderman Collection, Stanford University (Sorry about the long link)
http://collections.stanford.edu/images/bin/search/advanced/process;jsessionid=F275D8368A1C2E1DE618DD807F6E4005?clauseMapped%28collectionBrowse%29=The+Barry+Lawrence+Ruderman+Collection&sort=dateCreated&browse=1&sortReverse=false&offset=0
6992 maps dating from 1482 or earlier. First several pages are unknown date maps, then the pages go chronologically.
Getting a little closer to home - Kansas City Public Library, Missouri Valley Secial Collections
http://www.kchistory.org/cdm4/browse.php?CISOROOT=/Maps
861 maps, not all online, not in any particular order, and not near as user friendly of a viewer.