This is a treemap of every county (or county equivalent) in the United States, where the area of each rectangle is proportional to the population of the corresponding county.
Counties that are part of the same core-based statistical area are grouped together. (These have a thicker green border.) Moreover, core-based statistical areas that belong to the same combined statistical area are grouped together. (CSAs have an even-thicker red border.)
For example, the red rectangle in the upper-left corner represents the New York-Newark, NY-NJ-CT-PA CSA:
Check out the interactive version, where you can mouse over each rectangle and get information about that county and any statistical area of which it is part.
Later, I will post the code I used to make this treemap.