Khan Academy offers free online courses with video lessons and practice exercises in mathematics, physics, chemistry and other subjects.
Desmos is a free online graphing calculator and mathematical visualization tool for exploring functions and equations.
Dictionary of mathematical terms and definitions with brief explanations.
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.
Free open-source software for symbolic mathematics, algebra, and numerical computation.
Problems and solutions from the International Mathematics Competition for university students.
Paul's online course notes covering algebra, calculus, and differential equations.
Website with interactive explanations, animations, and simple problems covering elementary to secondary mathematics.
Official IMO 2025 Australia site hosting competition problems and the shortlist.
IMO Math is an archive of International Mathematical Olympiad competition problems and solutions.
Online database of mathematical objects used in number theory and related fields.
American Mathematical Society; a professional membership organization for research mathematicians.
Online book on group theory with visual diagrams and structured explanations.
A personal website with notes on mathematics and related topics.
Personal website with articles and resources on mathematics, polyhedra, and geometric sculpture.
European Mathematical Society press publishing peer-reviewed mathematics journals.
Russian portal with mathematics textbooks, problem collections, and reference materials.
A collection of geometry competition problems from Russian mathematical olympiads.
Archive of problems and solutions from Russian mathematics olympiad competitions.
Russian-language archive of mathematics theory explanations and solved problems.
Collection of problems from the Möbius math contest for students.
Saint Petersburg mathematics center providing lecture notes and teaching materials on mathematical methods and theory.
Russian youth mathematics school site offering problems and materials; status and content unverified.
Archive of geometry olympiad problems for secondary school students.
Online archive of Moscow Mathematics Olympiad past problems.
MateOnline offers solved mathematics problems and lessons online, covering algebra, geometry, and calculus.
Online repository of digitized mathematical literature and papers.
Archive of Brazilian Mathematical Olympiad competition problems and solutions.
Archive of mathematics olympiad problems (not physics); not relevant to physics students.