It should be noted that nearly everyone who does well on the USAMO has studied supplementary resources such as the Art of Problem Solving series, since the topics covered on the exam are not taught in most standard high school mathematics curricula. Recently, the contest has been growing increasingly competitive as more and more contestants were becoming aware of the availability of these resources.
Here are the subjects on the test in different years by problem number (ie what subject each problem was from):
2006:
1. Number Theory
2. Algebra/Combinatorics
3. Number Theory/Algebra
4. Algebra
5. Game Theory/Algebra
6. Geometry
2005:
1. Number Theory/Graph Theory
2. Number Theory
3. Geometry
4. Geometry/Algebra
5. Combinatorics
6. Algebra
2004:
1. Geometry/Inequalities
2. Algebra
3. Geometry
4. Game Theory
5. Inequalities
6. Geometry
2003:
1. Number Theory
2. Geometry/Algebra
3. Algebra
4. Geometry
5. Inequalities
6. Game Theory