Technology makes it fun these days to compare maps of the past with their modern-day equivalents to learn just where the nearest cemetery or church may have been or why your ancestors went to the next county to record their family's deeds and vital events. Historical overlay maps, which have been available for Google Maps and Google Earth since 2006, make this type of cartographic research very fun and easy. The premise behind a historic overlay map is that it can be layered directly on top of current road maps and/or satellite images. By adjusting the transparency of the historic maps, you can "see through" to the modern-day map behind to compare the similarities and differences between old and new maps, and study the changes in your selected location over time. A great tool for genealogists!
Hundreds, and more likely thousands, of organizations, developers, and even individuals like you and me have created historic overlap maps for the online tool Google Maps (nice for people who don't want to download the Google Earth software). 120 historical maps from the David Rumsey Map Collection, for example, were integrated into Google Maps last year. Additional historic map overlays you might want to explore include North Carolina Historic Overlay Maps, Scotland Historical Map Overlays, Henry Hudson 400, Greater Philadelphia GeoHistory Network, and Singapore Historical Maps - This isn't an overlay, but combines a map with historic photos of Singapore places in an interesting way.
If you really love these historic overlay maps, you may want to download the free Google Earth software. There are many more historic map overlays available through Google Earth, than through Google Maps, including many posted directly by Google. You can find the historical maps in the sidebar section titled "layers. Here are some tips to help you get started working with historic overlay maps.