Khan Academy
Khan Academy is 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. Khan Academy's materials and resources are available to you completely free of charge. Students can make use of their extensive library of content, including interactive challenges, assessments, and videos from any computer with access to the web.
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Excellent resource.