Khan Academy

Began as a series of video lectures in mathematics, goal is now to cover a range of topics at K-12 and college level. Khan has also branched out to tutorials/formative assessment exercises, currently mostly in math -- these contain links to associated videos where students can watch worked examples. Instructors can set up a "coach" account, and have students sign up to view their progress. See Salman Khan's TED talk for an explanation of where the idea for the original videos came from and how teachers use these resources. Khan Academy also includes some limited videos and exercises for the Humanities and Standardized Test prep.