I've seen a fair few map comparison sites over the years, map applications that allow you to compare one country with another country or one city with another city. BBC Dimension, OverlayMaps, MAPfrappe and If It Were My Home? are four good examples of apps that allow you to compare one location with another.
All four of these maps work by overlaying the shape of one location on top of another so that you can make a direct comparison of size. Mapmerizer works a little differently as it lets you compare two locations side-by-side on two different maps. Type in the name of two towns, cities or countries into Mapmerizer and you compare a Google Map of each location juxtaposed next to each other.
One neat feature of Mapmerizer is that you can also compare the satellite views of your two different locations. For example, when comparing the satellite views of London and New York, London seems much greener than the Big Apple, while in New York the population appears to be more densely concentrated. Source