Wolfram MathWorld is an online mathematics encyclopedia covering algebra, calculus, geometry, and related topics with definitions and formulas.
Khan Academy offers free online courses with video lessons and practice exercises in mathematics, physics, chemistry and other subjects.
GeoGebra is a free platform for creating and exploring interactive mathematics visualizations and geometric constructions.
Desmos is a free online graphing calculator and mathematical visualization tool for exploring functions and equations.
Seeing Theory is a visual and interactive introduction to probability and statistics concepts with interactive chapters.
Evan Chen's site with olympiad problems, solutions, teaching notes, and contest archives for mathematics olympiad preparation.
Dictionary of mathematical terms and definitions with brief explanations.
A wiki collecting mathematical and physics problem-solving methods and tricks.
Large collection of essays explaining mathematical and physical concepts from first principles.
Online encyclopedia of mathematical definitions and results.
PlanetMath is a collaborative online encyclopedia of mathematics entries written and peer-reviewed by community members.
Collection of expository mathematical papers and course notes.
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.
Mathematics problems and articles from Cambridge's NRICH and Plus platforms, covering applied mathematics and problem-solving.
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.
Large interactive collection of mathematical problems and proofs.
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 collection and database of mathematical proofs organized by topic.
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.
A comprehensive searchable database of Russian olympiad problems across many subjects with solutions.
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.
Problems from the Tournament of Towns mathematical olympiad, accessible to strong secondary and early university students.
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.
Aggregated physics and mathematics olympiad problems from regional and republican competitions in Kazakhstan.
Russian youth mathematics school site offering problems and materials; status and content unverified.
Russian portal for mathematics education containing problems, methodologies, and reference materials.
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.
These are filed under other headings but serve this one too.