Found this cool chart on the Economist's website showing which countries are the GDP equivalents of which US states, i.e. Maine's GDP is $51.29bn and its equivalent country is Luxembourg.
Some interesting ones are Nevada, Illinois and Idaho. Remeber these are not per capita so Oregon having the same GDP as Pakistan is rather impressive when you consider the differences in population.
Its an interesting exercise, more in terms of highlighting the differences facing the nations and their US GDP equivalents rather than what they have in common. For example, a California default would be problematic but manageable for the US Fed, however a default by Italy (perhaps only slightly less likely!!!) would probably lead to a meltdown in the Eurozone and a huge crisis in the sovereign debt markets.