From its website
The Khan Academy is an organization on a mission. We’re a not-for-profit with the goal of changing education for the better by providing a free world-class education for anyone anywhere.
All of the site’s resources are available to anyone. It doesn’t matter if you are a student, teacher, home-schooler, principal, adult returning to the classroom after 20 years, or a friendly alien just trying to get a leg up in earthly biology. The Khan Academy’s materials and resources are available to you completely free of charge….
Our library of videos covers K-12 math, science topics such as biology, chemistry, and physics, and even reaches into the humanities with playlists on finance and history. Each video is a digestible chunk, approximately 10 minutes long, and especially purposed for viewing on the computer.”
For people accustomed to learning from videos, Khan Academy could be a godsend because one can play the videos and review resources, ask questions and offer tips to co-learners in discussion forums associated with videos, anytime you want, as often as you want, as many times as you want. Your progress is tracked in your profile to document your learning accomplishments. As you complete more units of study, points are added to your profile and if you accumulate enough points, your initial avatar is upgraded to progressively new images to signify your attainment levels. You can also accumulate badges proclaiming your mastery from apprentice to master levels of achievement. Khan Academy has over 4,000 videos and offers instruction in math, science and economics, computer science, humanities, and college test preparation. It’s free.