It's not a plugin. It is a bunch of development that is heading towards being included in the Moodle 2.1 release. You can see the latest code at https://github.com/timhunt/Moodle-Question-Engine-2 but I would not recommend using it for anything other than testing until Moodle 2.1 is released, which is supposed to be on 30th June.
Just to give a progress update. The tracker issue to watch is MDL-20636. I have basically two more tasks to finish next week before the deadline of 31st May for getting new features finished if they are to go into Moodle 2.1.
That is, converting the multianswer and randomsamatch question types to work with the new system. For comparison, I converted the three calculated question types this week. Then there are a bunch of minor things to fix during the testing period between 31st May and 30th June, plus whatever bugs are found during testing.