A slightly out of date list of those countries that now have "open skies" relationships with the USA, arranged by geographic region, is available here on the Department of Transportation web site. The US Department of State list, which is in chronological order, is more up to date.
It has got to the point where a more interesting list would be of the countries that do not have "open skies" agreements with the United States. Notable absentees include Brazil, China, Colombia, Egypt, Mexico, Russia, Saudi Arabia, South Africa, Turkey and Vietnam. A 13 April 2010 article in BTNonline.com quotes John Byerly, State's Deputy Assistant Secretary for Transportation Affairs, commenting on the next negotiating priorities for the United States.