Here are some links to Mapping and GIS resources that have been useful to me
Maryland State archives: Plats of lots west of Fort Cumberland allocated to soldiers who
served in the Continental Army during the Revolution and of
tracts patented, purchased, or settled prior to 1787. In that
year Francis Deakins by appointment from the Governor and Council
surveyed those lots and tracts and prepared a large map to
portray them. In 1874 the General Assembly authorized a revision
of Deakins' map (Ch. 322) to remedy omissions and provide
additional information. The resulting product, directed by the
Land Office, numbered all military lots, showed topography, and
indicated land escheated to the state since 1787. LINK
Western Maryland's Historical Library: Nice summary of the military lot maps and links to the original 1787 map at the Library of Congress
Top 10 Jewels of the FSU Library Map Collection: Mary Jo Price selected a “Top Ten” based on
uniqueness in their content, condition, and age, as well as on their value to regional
history and geography. Gives a good overview of maps available there.
University of MD Library Digitized Map Collection:Some good 19C maps of MD. Free download. LINK
University of Alabama Map Project: Lots of great historical maps of every state. Digital copies are cheap. Alabama Maps is an ongoing project of the Cartographic Research
Laboratory, which operates under the auspices of the College of Arts and
Sciences at The University of Alabama.
State of MD Dept of Planning's Property Map Products: Say what you like about Maryland's regulatory and tax environment (and I have plenty to say), but they have excellent public data available. MD's Property Finder provides either the raw GIS data for you to manipulate or has a number of online viewing platforms, too. Data includes tax parcels, imagery, and numerous planning layers.
Garrett County's Dept of Planning's WebMaps: Our own county has an excellent Web Map application where you can overlay tax parcels, make measurements and look at a variety of overlays and base maps. A great resource for orienting yourself in the county and snooping on your neighbors!
Mapping Maryland's Counties: The selection of maps in this collection provides snapshots of the growth and development of Maryland's counties and the City of Baltimore. Collection in the Enoch Pratt Free Library
Below are full sized maps used to generate the slides in the History of the County presentations above. They are big files in order to preserve as much of the visual detail as possible, and also contain clickable layers with modern-day features like roads and current property lines. The file on the end goes over to work with pdfs with clickable layers if you've never worked with them before.