The United States of America Mathematical Olympiad (USAMO) is a prestigious high school mathematics competition held annually in the United States. Since its debut in 1972, it has served as the final round of the AMC series of contests. Top scorers on the USAMO usually represent the United States at the International Mathematical Olympiad.
Website: http://www.unl.edu/amc/e-exams/e8-usamo/usamo.shtml