Taxes per Person
The most common metric for answering this question is taxes as a percentage of GDP. However, high tax rates tend to depress GDP. Looking at taxes as a percentage of GDP may mislead us into thinking we can increase tax revenue more than we actually can. For some purposes, a better statistic may be taxes per person, which we can compute using this piece of advanced mathematics:
Taxes/GDP x GDP/Person = Taxes/Person
Here are the results for some of the largest developed nations:
.461 x 33,744 = 15,556
.406 x 34,219 = 13,893
.390 x 35,165 = 13,714
.282 x 46,443 = 13,097
.334 x 38,290 = 12,789
.426 x 29,290 = 12,478
.373 x 29,527 = 11,014
.274 x 32,817 = 8,992
The bottom line: The United States is indeed a low-tax country as judged by taxes as a percentage of GDP, but as judged by taxes per person, the United States is in the middle of the pack.
Update: This post has been more controversial than I expected. I am surprised because I did not say much here. I merely presented an identity and some data, which illustrated international differences in a novel (and, I thought, interesting) way. In any event, I thank Scott Sumner for coming to my defense.