Online courses, and massively open online courses (MOOCs) in particular, are quickly moving from a novelty to the mainstream. Leading colleges, graduate schools, and companies are investing significant resources into free education.

There are now hundreds of free courses available online, which can vary significantly in quality. That makes it essential to pick the right course.

We've found some of the most fascinating, highly regarded, and useful courses around, taught by some of the most accomplished professors at the best schools in the world.

If you take a class through a platform that runs more structured courses, like EdX or Coursera, it can be beneficial to take them in session. Then, you can get assignments graded, occasionally have access to teaching assistants and even professors, and have a community of other people taking the course that you can talk to and rely on. And if you do well enough, you'll receive a verified certificate for completing the course.

Some of the following courses are running right now, while others have already concluded but can still be taken as self-paced courses. We've noted which is the case for each.