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edX is an organization that aims to make educational resources more accessible to the general public. They do so by providing free access to most courses, although if you are looking for graded assignments and certificates of completeion, you'll have to pay.
edX has over 2,000 courses that all provide a certificate of completion upon following the 'upgraded' path. You can get free access to pretty much the whole catalog and can get certficates and graded assignments for payments that range up to $99.
Want to know more about edX? Check out the video on the right.