Google has just launched the Google Maps Gallery to help “unlock the world’s maps”. It’s an impressive collection of maps of various kinds, easily accessible in one place.
The maps all have a “View in Google Earth” button to generate a KML for your use, and some of them (such as this Tokyo 1680 map) have slick transparency sliders to make them more useful. As Keir Clarke at Google Maps Mania points out, the maps are all embeddable as well and makes it somewhat like a “YouTube for maps”.
There are some great maps in there and I strongly encourage you to check it out. You can browse them all at maps.google.com/gallery/ or read more about it over on this post on the Google LatLong Blog.