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.
Wolfram Mathworld is an extensive resource for definitions and descriptions of various math topics arranged by subject.
Visit The World of Math Online for definitions and practice problems for all math subjects up to and including Calculus.
Quizlet contains free learning tools for many subjects! There are flash cards, automatically graded tests, timed exercises, and interactive games.
Visit Paul's Online Math Notes for free online (and downloadable) notes and/or tutorials for math topics.