Mathematics & Statistics

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.