According to Wikipedia, Massive Open Online Course (MOOC) is an online course aimed at unlimited participation and open access via the web. These courses becomes very popular recently.
I took my first online course at the end of 2011 at Coursera. It was "Introduction to Databases" by Stanford University (this course will be back soon). I was afraid my knowledge of English will be insufficient for the course. But all was fine - I enjoyed the course and it help me with my university course on databases. Few other courses at Coursera was fun too. What makes Coursera unique is a quantity of courses, hundreds of it.
Then I loved Udacity. Their courses are very practical and exciting. I have completed 8 of them and partially watched almost all other courses. What I especially like is that all of these courses are self-paced: you can watch and complete them any time you want. But then Udacity enabled paid certificates and add a few courses developed with some corporations. Last one (on Hadoop) I finished only yesterday. Yep, it is still fun and I don't even bother about certificates, knowledge is more important. But I don't recognize Udacity: course was too short and cursory. I hope upcoming courses will be more interesting.
The last one platform I have used is Edx. Now I think this will become my favorite. It has a few courses compare to Coursera. But I like this platform. It is open-sourced and beautiful. Courses seems pervasive for me. I haven't finish one yet, but I watched few of them. I hope this platform becomes more popular and get more interesting courses.
For any interested in online learning I can recommend really cool MOOC tracker class-central. It seems to be the largest and the simplest one tool for planning your learning.
I would happy to see your opinions, remarks and experience on MOOC in comments. Comments on English will be appreciated too.