MIT is one of the best-known schools in the U.S. MIT Open Courseware is free access to material from 1,950 of MIT's courses.
While you can use any of this material to learn, on your own, it's important to realize that you do not get credit for your work, or a grade, and completion does not equal an MIT education. You get no degree or certificate. You get learning and edification.
There is no registration, and you can start and stop any course any time you want to.
Anyone who knows anything about technology knows that this kind of offering, while free to users, is not free to provide. MIT accepts donations and welcomes feedback.
MIT President Susan Hockfield has this to say about OCW, "There is no limit to the power of the mind. We encourage you to use OCW—learn from it and build on it. Find new ways not only to pursue your personal academic interests, but to use the knowledge that you gain—and that you create—to make our world a better place."