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/

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

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/

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

Math Competition on the Web  
   Links to national and international competitions as well as training pages for Putnam.
   http://donut.math.toronto.edu/~naoki/math-comp.htm

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

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/