Adherents.com has data on the top 50 countries with the highest percentage of people who do not believe in God (though may not self-report as “atheists”) and the top 20 countries with the largest number of nonbelievers. The data comes from Phil Zuckerman, “Atheism: Contemporary Rates and Patterns,” in Michael Martin’s The Cambridge Companion to Atheism (2006, Cambridge University Press).
The United States is #43 for percentage of nonbelievers, and #7 for absolute number of nonbelievers (after China, Japan, Russia, Vietnam, Germany, and France). The top ten countries for highest percentage of nonbelievers are Sweden, Vietnam, Denmark, Norway, Japan, Czech Republic, Finland, France, South Korea, and Estonia.
UPDATE (April 4, 2006): Added a link to the Zuckerman chapter, which is available online.