The other contest that I am often involved with is the International Mathematical Olympiad (IMO). At the invitation of the Canadian Mathematical Society, I assumed the role of Leader of the 2002 Canadian IMO team, Deputy Leader of the 1999 Canadian IMO team, and a Leader Observer for the 1998 Canadian IMO team. The IMO is an international mathematics competion for high school students. Currently, about 80 countries compete. For more information, visit the Canadian IMO site or the USA IMO site.
For practice materials for any of these exams, I recommend in particular visiting the American Math Competitions website. In particular, I would like to draw your attention to the following: