Evan Chen's site with olympiad problems, solutions, teaching notes, and contest archives for mathematics olympiad preparation.
A wiki collecting mathematical and physics problem-solving methods and tricks.
Art of Problem Solving hosts forums, problems, and discussion for high school and undergraduate math and science olympiad competitors.
Forum and archive for sharing and discussing olympiad-level mathematics problems across many competitions.
Academic notes and essays on mathematics, primarily combinatorics and competition topics.
Official IMO 2025 Australia site hosting competition problems and the shortlist.
IMO Math is an archive of International Mathematical Olympiad competition problems and solutions.