“NOT FOR NAVIGATIONAL USE” the site says in very large red letters. Also in red: “Facsimile prints and reprints of these historical maps and charts are NOT available.” Um, okay. If you can get past that, you can search a database of over 21,000 digitized maps and charts from 1747 to 2001. It’s available at http://historicalcharts.noaa.gov/historicals/historical_zoom.asp.
You can search by keyword, by state or region (these maps are international), type (from aeronautical chart to topographic index), year, or chart number. I did a search for maps in Maryland and got 813 results. Results come in a table and I can’t tell how they’re sorted. It does appear that all the results show on a single page, which is handy when you’re trying to find the oldest map, but it does mean you might have to wait a few minutes for it to load.
I finally settled on a State of Maryland nautical chart from 1840. The search results show the region that the map covers (in this case, MD-VA-DC-DE) and offer a preview link for the map. Click that and you’ll get a Flash-based Zoomify map that lets you zoom in and out, drag it around, etc. If you want to download the map you can get a SID version or a JPG. The downloads were free and didn’t require any registration.
If you’d like to explore these maps but can’t think of any queries, I suggest browsing the map types. There are 29 Civil War maps (including several troop movement maps), one “sketch” map, a couple of pocket maps, etc. Alas, there are not yet any City Plans marked as such in the database, and a couple of the other map types are empty too.