Although it is now more than a year old, National Geographic Magazine posted a very interesting map of the most common surnames on its blog (here). The map lists the most common surnames for each state and varies the size of each name according to the number of people who carry that name in the state and the color of the name according to its assumed origin.
Surname mapping tools can be very useful for figuring out what part of a country your ancestors might have come from. In previous posts, I have discussed mapping tools for Poland (here) and France (here). Some of these sites allow you to look at the data in different ways. Maps can show relative distributions (frequency per some number) or absolute distributions (total number for each geographic division). Some maps also allow you to look at the distribution of a surname during different periods.
A list of tools for different countries is given on the Surname Tools page on this blog. I will add more sites as I find them.