From the about page:
Education is changing, with great educators from around the world increasingly putting their amazing courses online. We believe we are in the early days of a revolution that will not only increase access to great education, but also transform the way people learn.
SlideRule is our way of contributing to the movement. We help you discover the world’s best online courses in every subject – courses that your friends and thousands of other learners have loved.
I counted thirty-three (33) content providers who are supplying content. Some of it free, some not.
It looks extensive enough to be worth mentioning.