MIT OpenCourseWare
ocw.mit.edu
Free lecture notes, exams, and videos from over 2,500 MIT courses.
MIT OpenCourseWare publishes materials from virtually all MIT courses for free. It includes lecture videos, problem sets, exams with solutions, and reading lists — covering everything from physics to philosophy.
Why it is useful
Access to an elite university curriculum with no enrollment or payment required. The problem sets and exams with solutions provide structured self-study that most free resources lack.