A few days ago I made the Enhanced Relation List publicly available. It's currently in the stable version 1.0.0 and got some improvements compared to the previously mentioned prototype.

The configuration got slightly expanded in a way to give more control over what the field will allow an editor to do. An example would be the selection of one content item multiple times. This "feature" was a bug within the prototype and has been set to be configurable (you can allow and disallow duplicates).

It is still close bare bone UX but (in my humble opinion) still a good one.

To get a comprehensive overview of the features available, have a look at the documentation over at github.