Cartograms

http://en.wikipedia.org/wiki/Cartogram

Cartograms are a thematic map that utilizes color to portray some variable in different areas. This variable could be GDP, populations, or travel time. They date back to the mid-1800s, and have a wide variety of uses. They can be used to gain a better knowledge of a certain variable in different areas and the colors allow you make connections between different regions.