Desmos is a mathematics website that has two main features. The first of these features is an online graphing calculator that is both highly advanced and free to use. The other, less known feature is its set of classroom activities designed to help students explore the world of mathematics.
Khan Academy is a site created by Salman Khan to educate people of all ages in all sorts of topics, but mainly math. Khan Academy teaches math through the use of lectures, examples, and practice problems, just like any school, and is completely free to use.