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.