It is said in the edX website that edX is an open source platform but I cannot find the website anywhere. I have checked also SourceForge without success.
closed as not a real question by Al Everett, ChrisF♦ Mar 29 at 11:45
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I believe you can't find it because it isn't available yet. An original press release (July 2012) mentions:
And more recently in an article (Sept 2012) about MongoDB getting involved:
The edX homepage still lists jobs for platform developers, so looks like it is still a work in progress. In the meantime Google has released an experimental Course Builder platform, or there is the popular Moodle.
XBlock courseware component architecture was recently released and is available at https://github.com/edX/XBlock