When you first started working on your family tree, you probably didn't think a lot about using maps as a genealogy resource. I know I didn't. However, as you spend more time tracing your ancestry, you will find that maps can a helpful tool in more than one way.
First, maps can help you determine border changes. In many cases, whether moving forward or backwards through census records, disappearing ancestors are often the result of a border change. They probably have stayed in the exact same place, but that place may be part of a different county or state in that particular census. West Virginia was part of Virginia until 1863. Tennessee was initially part of North Carolina.
Second, maps can help you determine factors that may have affected the interactions of your ancestors. For instance, if you know the place where your ancestor was born was a remote valley in the mountains and transportation options were limited to horses, you can probably assume that (a) he stayed in the same area for most, if not all, of his lifetime unless there was a compelling reason to leave the area, (b) he probably married, raised a family and died in or around that area and (c) his spouse was from that area as well.
There are many places you can find historic maps online for use in tracing your ancestry. Many of these can be added to your free genealogy resources toolbox.
The University of Alabama Historic Map Archive
One feature I really like about this archive is that it isn't limited to only the United States. You can find maps for locations worldwide. There are also maps for special topics such as the Revolutionary War, Native Americans and railroads, just to name a few.
The United States Digital Map Library
A project of the USGenWeb, this collection has a variety of maps contained in three sections. The first section, US Maps, has maps of the US and specific regions of the US through time. The second section, State and County Maps, offers more, including specific ones for conflicts and railroads. Some states offer more than others. The final section is the Indian Land Cessions to the United States.
The Hargrett Historic Maps Database
This database is part of the University of Georgia's Rare Book & Manuscript Library. You can search by keyword, title, author or year range or browse by title, creator, facet or century. Not all maps listed will have a digital version available, but many do.
The Library of Congress American Memory Map Collections
This free genealogy resource shows a small portion of the Library of Congress' Geography and Map Division. Don't be mislead by the use of the word small. The entire collection consists of over 4.5 million items. The collection is divided into seven sections-Cities and Towns, Conservation and Environment, Discovery and Exploration, Cultural Landscapes, Military Battles and Campaigns, Transportation and Communication, and General Maps. You can also search by keyword or browse by geographic location, subject, creator or title.