The edX modulestore stores the course content for all versions of all courses and is backed by MongoDB.
http://edx.readthedocs.io/projects/edx-developer-guide/en/latest/modulestores/index.html
https://github.com/edx/edx-platform/wiki/Mixed-Modulestore
Why/how does the modulestore use MongoDB? This presentation might help:
http://doctoryes.github.io/mug_talk_modulestore/#1