American Mathematics Competitions
   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.
   http://www.unl.edu/amc/

Australian Mathematics Trust
   Sponsors a number of competitions in the Australian continent.
   http://www.amt.canberra.edu.au/

Crux Mathematicorum with Mathematical Mayhem
   Problem-solving journal at the senior secondary and university undergraduate levels.
   http://journals.cms.math.ca/CRUX/

Math Competitions in USA
   Sample problems, information and links from multiple contests.
   http://www.albanyconsort.com/contests/contests.html

Mathematics Archives
   links to contests, problem sets, journals sites.
   http://archives.math.utk.edu/contests

Putnam Exam Information
   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.
   http://www.math.harvard.edu/putnam/

Valentin Vornicu's MathLinks
   Math Olympiad Problem Solving site with a weekly contest, resembling IMO in difficulty, with a large math forum and downloadable materials.
   http://www.mathlinks.ro