Putnam Exam Information - http://www.math.harvard.edu/putnam/ Results from the Putnam competition, old exams, and problem archives. Includes problem of the day, selected from over 540 different Putnam, National or International Olympiad problems.
Australian Mathematics Trust - http://www.amt.canberra.edu.au/ Sponsors a number of competitions in the Australian continent.
Math Competitions in USA - http://www.albanyconsort.com/contests/contests.html Sample problems, information and links from multiple contests.
Mathematics Archives - http://archives.math.utk.edu/contests links to contests, problem sets, journals sites.
American Mathematics Competitions - http://www.unl.edu/amc/ Organization that administers the AMC, the AIME, and the USAMO, with information about the contests, math problem archives, recommended reading lists, and other resources for students interested in mathematics.
Valentin Vornicu's MathLinks - http://www.mathlinks.ro Math Olympiad Problem Solving site with a weekly contest, resembling IMO in difficulty, with a large math forum and downloadable materials.
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