MIT OpenCourseWare publishes lecture notes, problem sets, exams, and sometimes video lectures from thousands of MIT courses, all free to access. The materials go back decades and cover every department the university teaches: physics, mathematics, computer science, economics, architecture, and more. You don't get credit or interaction with instructors, but you do get the same readings and assignments that MIT students work through. For self-directed learners, it's one of the most substantial academic libraries on the open web.

The site organizes courses by topic and level, and many include complete video lectures from the original class recordings. Some courses are more complete than others — a few have everything except the exams, while others are just a syllabus. It's not a structured degree program, but if you want to study a subject deeply and don't mind working without a teacher, it's hard to find a better free resource.