Maps can show where your family lived, and who their neighbors are. Because people would move from one place to another in family groups, you may find even more family members. Here is an example from the American Memory Project http://memory.loc.gov/cgi-bin/query
Here are a few other locations http://www.csginc.org/ct_genealogy_maps.php http://ontariogenealogy.com/ontariomaps/uppercanadaeastmap.jpg
http://www.libraryspot.com/maps/historical.htm http://www.lib.berkeley.edu/EART/browse.html
Another imporant fact about where your family lived is the movement of city and county boundary lines. The location where your ancestor lived may have had its boundary changed, and so you may have to go to a different community to find vital records. http://www.familyhistory101.com/map_census.html
Sanbourne maps (1867-1970) are insurance maps developed to help fire insurance providers, particularly in rural areas. Information about businesses is provided--the size of the building, its construction, its location on the street block. Some of these maps are available online, and can provide some interesting information about your ancestor's business ventures. Here are a few examples.
http://palmm.fcla.edu/sanborn/
http://olc1.ohiolink.edu/search/X?SEARCH=sanborn+maps&SORT=D
http://libraries.mit.edu/guides/types/maps-urban/sanborn-boston.html
http://dlg.galileo.usg.edu/sanborn/?Welcome
Plat maps are also a rich source of family information. They show the location of where you family lived, the size of the plot of land, and your family's neighbors. Often neighbors are other family members, increasing the information you can find about your family. Often, you will need to write to the library in the community where your ancestor lived to get a copy of a plat map, but there are some available online.
http://www.info.co.clay.mn.us/History/clay_county_plat_map_1909.htm
http://digicoll.library.wisc.edu/WI/subcollections/Walworth1857About.shtml
http://digicoll.library.wisc.edu/cgi-bin/WI/WI-idx?id=WI.PlatBookDoor23
http://www.beforetime.net/iowagenealogy/mahaska/platmap1905/index.html
http://www.kinexxions.com/platmap/