H5P is an open source platform for creating rich, mobile-friendly HTML5 objects and exercises. They have an extensive list of content types to draw from, which all make nice additions to the existing components in Open edX.
If you’re interested in trying it out, you can get a free trial of H5P as-a-service, and we even provide an Open edX to allow testing.
(I would have gladly provided links to all these resources but new members on this forum can’t have more than 2 links in their first few posts. @nedbat might you consider giving members a higher trust level by default?)
Your comments and questions are most welcome!