https://www.khanacademy.org/
A free website aimed at promoting self-paced instruction, houses thousands of academic videos that are baked into guided, adaptive instruction. With an initial focus on math, the site now offers courses on a variety of topics in science, economics, history, the arts, language arts, computing, and test prep; course offerings are continually updated and expanded.
https://www.desmos.com/
Desmos is an online graphing calculator designed to help individuals visualize and understand mathematical concepts, particularly in the field of graphing and algebra.
https://www.purplemath.com/
Purplemath is an online resource that provides free math lessons and educational materials primarily focused on algebra.
https://www.mathsisfun.com/
Mathsisfun is an educational website that is well-regarded in Australia for its clear and engaging explanations of various mathematical concepts.
http://hyperphysics.phy-astr.gsu.edu/hbase/index.html
HyperPhysics is an exploration environment for concepts in physics which employs concept maps and other linking strategies to facilitate smooth navigation.
http://hyperphysics.phy-astr.gsu.edu/hbase/hmat.html#hmath
HyperMath is a growing collection of examples of applied mathematics with links to their applications to problems in physics and astronomy.