Mathematics Enrichment::NRICH Home Page: UK website with a wealth of mathematics resources for all levels. Monthly format but access to the archives in the sidebar. Searchable by topic, level, etc.
Living Math!: A wealth of resources for learning mathematics through literature and games.
Free Online Graph Paper / Grid Paper: includes options to specify size of squares, also triangles, hexagons, and other types of grid
Doc’s Anti Textbook Math Page! Long list of useful math stuff.
Purple Math: Algebra concepts explained. Good visuals.
Radical Math: I haven’t checked this out much but it looks interesting. Resources for integrating Math education with Social Justice Education.
Statistics Canada Learning Resources: Not just a source of statistics, lesson plans for teaching data management, etc. but also very useful for statistical data for other subjects (e.g. social studies, history)
Mathematical Fiction: looks like a pretty big database of titles over there
MathsNet: UK site with curricula, puzzles, etc.
National Library of Virtual Manipulatives: American site with virtual manipulatives for mathematics teaching across all grade levels.
Platonic Realms: puzzles, history of mathematics, games, etc.
The Art of Problem Solving: Books, online classes, forums, etc. for gifted math students
Devlin’s Angle: monthly columns on mathematics for the MAA
Mathematics Illuminated: from their description “Mathematics Illuminated is a thirteen-part series for adult learners and high school teachers. The series explores major themes in the field of mathematics, from humankind’s earliest study of prime numbers, to the cutting-edge mathematics used to reveal the shape of the universe.”
Math Playground: games, puzzles, and videos explaining concepts. K-8