Hypercities is a great collection of historical maps that can be viewed on Google Maps. The site now has historical map collections for 18 cities around the world.
If you select a city from the map the historical maps available for the city will load in the map sidebar. If you select a map from the sidebar you can view the map overlaid on a Google Map of the city.
The transparency of the historical map overlays can be adjusted with the slidebar control. It is therefore possible to compare the historical map with the Google Map underneath.