From Citizendium, the Citizens' Compendium
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- Online "Encyclopaedia of Mathematics" from Springer. Graduate-level reference work with over 8,000 entries, illuminating nearly 50,000 notions in mathematics.
- Interactive Mathematics Miscellany and Puzzles — A collection of articles on various mathematical topics, with interactive Java illustrations at cut-the-knot
- Some mathematics applets, at MIT
- Rusin, Dave: The Mathematical Atlas. A guided tour through the various branches of modern mathematics.
- Stefanov, Alexandre: Textbooks in Mathematics. A list of free online textbooks and lecture notes in mathematics.
- Weisstein, Eric et al.: MathWorld: World of Mathematics. An online encyclopedia of mathematics.
- Polyanin, Andrei: EqWorld: The World of Mathematical Equations. An online resource focusing on algebraic, ordinary differential, partial differential (mathematical physics), integral, and other mathematical equations.
- Planet Math. An online mathematics encyclopedia under construction, focusing on modern mathematics. Uses the GFDL, allowing article exchange with Wikipedia. Uses TeX markup.
- Mathforge. A news-blog with topics ranging from popular mathematics to popular physics to computer science and education.
- Metamath. A site and a language, that formalize mathematics from its foundations.
- Mathematician Biographies. The MacTutor History of Mathematics archive Extensive history and quotes from all famous mathematicians.
- Cain, George: Online Mathematics Textbooks available free online.
- Math & Logic: The history of formal mathematical, logical, linguistic and methodological ideas. In The Dictionary of the History of Ideas.
- Nrich, a prize-winning site for students from age five from Cambridge University
- MatheMagic, a site devoted to the popularisation of mathematics, and to "Maths for All"
- 'Count On' The sequel site to www.mathsyear2000.org
- Web-based LaTeX equation editor
- LaTeX search - an online tool to search through LaTeX code snippets in Springer's database of published papers
- Imaginary - an annotated gallery of illustrations of mathematical concepts, featuring curves, symmetry, chaos, fractals, Moiré patterns, kinematics and simulations
- Math Overflow — and interactive site in which mathematical questions are discussed
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