David Rumsey historical maps collection. This morning, through Google Maps Mania, I learned of another good place to find historical maps.
Historic Map Works is an online gallery of hundreds of historical maps. On Historic Map Works you can browse for maps by continent, country, state, and province. Contrary to my initial experience, downloading the map images is not free. But, you can view more than half of the maps as Google Maps overlays using Historic Map Works's free Historic Earth Basic.
Applications for Education
Historic Map Works could be a good place for students to find old maps to compare with current map views. One of the more interesting sets of maps on Historic Map Works is the Antiquarian maps collection. Have your students look through that collection to see how cartographers drew the world in the 14th and 15th Centuries.
Another good historical maps tool that I like is History Pin.
Post edited to clarify that downloading map images is not free, but viewing the maps is free.