Khan Academy

http://www.khanacademy.org/about

The Khan Academy is an organization on a mission. They are a not-for-profit with the goal of changing education for the better by providing a free world-class education to 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, or an adult returning to the classroom after 20 years. The Khan Academy's materials and resources are available to you completely free of charge. Students can make use of their extensive video library, practice exercises, and assessments from any computer with access to the web.