Project Euclid provides open access to journals in Math and Statistics, including Annals of Mathematical Statistics, Bulletin of the American Mathematical Society, and Statistical Surveys.

Curated list of textbooks evaluated and approved by the American Institute of Mathematics.

Activity-driven textbook created by faculty at Grand Valley State University.

Converts between 8 different bases.

Statistics concepts visualized, from Brown University.

Instead of pulling problems out of a database, Wolfram Problem Generator makes them on the fly, so you can have new practice problems and worksheets each time. Each practice session provides new challenges.

Khan Academy provides video lectures and accompanying practice problems in topics like differential equations, linear algebra, etc..

MIT OpenCourseWare (OCW) provides course materials for a wide variety of math subjects. Assets vary by course, but many courses provide practice problems and solutions.