There are a lot of sites on the internet that try to sell you old historical maps. Although I’m the first to say that if you want high quality products, you can’t expect to get them for free, some of these sites really deliver dubious products.
David Rumsey Historical Map Collection is a very nice exception to the above. It has high quality maps for countries all over the world. It also has detailed maps of cities from 1700 until now, and they can even be viewed in Google Maps and Google Earth. This is really good because you can see which streets and buildings still exist…
It’s an absolute delight to play with.