Khan Academy provides FREE videos with instruction, interactive challenges, and assessments. Find lessons on topics from math, science, economics, finance, and humanities.
MIT OpenCourseWare (OCW) is a web-based publication of virtually all MIT course content. There are video lectures, lecture notes, exams and solutions, and assignments.
​Quizlet contains free learning tools for many subjects! There are flash cards, automatically graded tests, timed exercises, and interactive games.