It is free, it doesn't require any registration. Obviously, you can't get any certificate or degree!
Others universities all over the world are sharing Course Materials: official ones are those affiliated to the Open Course Ware Consortium.
A great step towards education for all people is to make content and knowledge available for free and the smoothest way possible.
Contents are available as lecture notes, exams, videos, audio files...
It is also possible to contriute on this activity, translating contents in non-english languages or use them (giving the proper attribution to the original MIT Faculty Author/s).
Contents are released in Creative Commons BY-NC-SA License.