The Purple States of America
In How to Lie with Maps, Mark Monmonier discusses how maps can give false impressions. A perfect example of this is the map of the United States used to show "popularity". Many of the physically large states in the West have relatively smaller populations than the "smaller" states in the East.
Michael Gastner, Cosma Shalizi, and Mark Newman of the University of Michigan make an excellent demonstration of this cartographic "lie" with their analysis of voting in the 2004 Presidential Election. Some sample maps:
To look at a standard electoral map of the United States, it would appear that a significant majority of the nation is "red", or voting Republican. Look at all the "red" on the map!:
However, if you make a population cartogram (adjusting the size of the states by their relative population) you can see just how close the election really was:
Yet, is Ohio really all red, and Michigan really all blue? Of course not. So if you color the nation county-by-county, and give those counties different shades of colors between red and blue, based on how strong the vote was, you get something like this:
So, basically, after the 2004 election, the nation was one big bruise...
Read the full analysis and progression of 2004 presidential Electoral maps, or the author's breakdown of the 2006 Congressional elections, or more fun with international socio-economic cartograms as well as the World Mapper.