Gini Coefficient

http://www.businessinsider.com/chart-of-the-day-income-inequality-in-the-us-china-france-brazil-2011-10

From the site:

For those that don't know, the GIni Coefficient is a crude way of measuring inequality (You can see the math behind it here). The basic idea is that on a scale of 0-100, 0 would represent a country where everyone had identical amounts of money, and 100 would represent a world in which just one person had all the money and everyone else had nothing.