Khan Academy
The Khan Academy is an organization on a mission!

The academy is a not-for-profit resource 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 and absolutely free of charge.

It doesn't matter if you are a student, a teacher, a home-schooler, a principal, an adult returning to the classroom after 20 years, or a friendly alien just trying to get a leg up in earthly biology.

Let the exploration and learning begin!