OMDoc (Open Mathematical Documents; see the OMDoc book for details) was developed by Michael Kohlhase.
It can be seen as two things:
MMT fully subscribes to the OMDoc philosophy and can thus be seen as a concrete incarnation of OMDoc. Currently, MMT covers and heavily redesigns the formal subset of OMDoc (though work on the informal subset is currently under way).
Moreover, MMT has become a major driver of changes to the XML format or its specification.