This is a new Harvard Africa Map that allows for layering of different maps and geospatial materials from different collections. In this screenshot, I have layered two historical maps over the Google Map layer. You can even adjust the density of the maps off to the right. Quite a fun way to explore older data and a very promising creation.
You can find the beta site at http://cga-3.hmdc.harvard.edu/africamap/. Give it a whirl and enjoy.