Again, a very clean site with negligible ad presence. If you are in the mood to enlighten yourself and learn, or fill in the gaps on a college course you are taking, this is a free site that can help you.
Open Yale Courses are offered, not for college credit, but simply to learn. I took a 1 hour class on classical music that was very interesting and informational.
REASONS TO USE:
[1] You enjoy learning
[2] Negligible ads
REASONS TO LOSE:
[1] You don't like to learn.