MapofUS (only seems to work with Microsoft Edge browser, at least for me)
Atlas of Historical County Boundaries (The Newberry Library)
Alabama Maps (But not only maps of Alabama)
"The Historical Map Archive is a digitized collection of selected map holdings from sources including the University of Alabama Map Library, the W. S. Hoole Special Collections Library, the Birmingham Public Library Cartography Collection of the Birmingham Public Library, the Geological Survey of Alabama, Samford University Special Collections Library, and the Alabama Department of Archives and History. We are indebted to these organizations for their cooperation in facilitating the introduction of these digitial maps to the Internet."
Atlas of the Historical Geography of the United States (Digital Scholarship Lab, University of Richmond):
"Here you will find one of the greatest historical atlases: Charles O. Paullin and John K. Wright's Atlas of the Historical Geography of the United States, first published in 1932. This digital edition reproduces all of the atlas's nearly 700 maps. Many of these beautiful maps are enhanced here in ways impossible in print, animated to show change over time or made clickable to view the underlying data—remarkable maps produced eight decades ago with the functionality of the twenty-first century."
Blog for the Geography and Map Division of the Library of Congress
BLM GLO Records, also state land management links
Big Map Blog - "Interesting maps, historical maps, BIG maps."
FamilySearch Standard Finder
Historical Map and Chart Collection:
"Office of Coast Survey's Historical Map & Chart Collection covers the land and waters of the United States of America, including territories and possessions (past and present). The images are free to download, and may be used for commercial or educational purposes. Although not required, we encourage users to cite "NOAA's Historical Map & Chart Collection" when using the image(s)."Historical Soil Survey Maps ("The maps found in this collection must be accessed through the ExpressView (Mr.SID) plug-in. This viewer is free and available for downloading.")
David Rumsey Map Collection
Historic Map Works
HistoryGeo ($) see also Arphax Publishing for printed out original land owner maps
In Motion: The African-American Migration Experience
The Megalithic Portal: Prehistoric sites all over the world (I originally found this through Wikipedia's coordinate/Geo Hack function).
National Historical Geographic Information System (NHGIS)
agreed to the following terms:
Redistribution: You will not redistribute the data without permission
Citation: Cite the NHGIS data appropriately
Norman B. Leventhal Map Center
Se eiendom (Norway)
Open Street Map
Perry-Castañeda Library Map Collection
USGS Historical Topographic Map Explorer
What Was There
Map a List (also a blog post from Worldwide Genealogy about it)