A little-known resource from the Civil War era are the Gilmer Maps. Jeremy Francis Gilmer was was the Confederate Chief of Engineers. He made extremely detailed maps of various Confederate counties and cities, especially those in Virginia. I have always known about the collection of maps at the Virginia Historical Society. The VHS is pretty protective of them but offer to sell you copies. They also restrict your ability to reproduce and use the images.
The major importance of using Gilmer maps for genealogy is that they show the layout of farms, homes, trees, streams, railroads and even give the name of a homeowner. You can cross-reference the 1860 and 1870 censuses and figure out where your ancestors lived. (You can also use land tax records). I have found a lot of my people on these maps. In the above example, you will see a Dr. Lett and a J. Lett (where Lett looks like Bell). Dr. Lett is a cousin and J. Lett was my great-great-great-grandfather.
Examples of Gilmer maps can be seen here:
I recently discovered another batch of Gilmer maps, available online for free. They are housed at the UNC University (Wilson) Library, obviously in North Carolina. Below you will find links to the available online maps and to the greater Gilmer papers collection. I have not had time to browse all of the papers, or even all of the maps, so if you find anything interesting, please let us know.
Gilmer Civil War Maps Collection (online)
Inventory of the Jeremy Francis Gilmer Papers, 1839-1894