Firstly, the program is called edX, and here is a link to their main website link: https://www.edx.org/
Their partners (the schools offering courses) include, but are not limited to: Harvard, MIT, UC-Berkeley, University of Texas, Boston University, Australian National University, Georgetown University, The University of British Columbia, Arizona State University, Notre Dame, Princeton, Cornell, and many more
Second, Some of their courses go on a set schedule (some go over the winter, some over the summer), and others are self-paced. In fact, if you are uninterested in credit or certification, you can even take older classes at any time. check out this link for more information about how it works: How It Works!
Third, classes are setup based on their certificate/credit offerings. From what I can tell, the vast majority, if not all classes, can be audited for free (meaning you can take any class they offer for free). Some of these classes will offer free certificates, but these "open" or "honor code" certificates do not verify your identity. If you verify your identity, then you can qualify for a verified certificate. These certificates will contain your name, the school's logo (such as Harvard), and the names/signatures of the professors. A verified certificate costs money, but very minimal (Most verified certificates are only $49, others are $90). Some classes offer actual college credit (these cost around $600). However, these classes also qualify for financial assistance that cover 90% of costs. You can take the class, and apply for financial assistance while taking the class. (a credited class could cost as little as $60, a verified certificate could become free or cost as little as $5-$9). Here is the link for financial assistance: edX Financial Assistance
Here is an image of a sample certificate:

Finally, here are some sample courses that will begin soon that I might be of particular interest to my fellow Ninjas:
Computer Graphics - UC-San Diego
Introduction to Computer Science - Harvard
Introduction to Game Design - MIT
Climate Change: The Science - The University of British Columbia
HTML5 Part 1: HTML5 Coding Essentials and Best Practices - World Wide Web Consortium (W3C)
Introduction to Solar Systems Astronomy - Arizona State
If you have any questions that you cant get answered via the website, then feel free to ask me on this thread. I will do my best to help.