Problem Archives
20,000 Problems Under the Sea, Arkadii Slinko's Math Olympiad, IMO Problems, International Mathematical Talent Search, Internet Mathematics Contest Problems, Kalva, Kiran Kedlaya's Math Problem Directory, Madras College Mathematics Department Resources, Math Olympiad Problems from Around the World, Math Olympiads
Greg Martin (UBC)
Syllabus and practice problems from Rajiv Gupta and Greg Martin's Putnam practice sessions.
Madras College Mathematics Department Resources
Assortment of materials including British Mathematics Olympiad papers and Mathematics Higher Course Notes.
Mathematics Olympiad Lecture Notes
By Greg Gamble for highschool students intending to participate in Mathematics Olympiads.
Problem Solving References & Resources
Overview of seminars oriented towards problem solving. Includes sample problem sets, list of online resources, college mathematics journals, and useful books. Maintained by Andrew Bernoff (Harvey Mudd College).
Ravi Vakil (Stanford)
Training resources for the Putnam competition. Practice problems, notes on mathematical theories, and recommended reading.
Tutorials in Elementary Mathematics for Math Olymp
Series of tutorials to cover the material of the unwritten syllabus of the IMO.