DFX Monomaker

A very simple plugin that allows you to merge stereo channels to mono. You can "narrow" the stereo image as much or as little as you like. The result can also be panned by monomaker.
Documentation is unnecessary because it's so simple. The interface is nice enough and it's perfectly clear what the controls do (there are only two).
It's a very useful little utility that can come to the rescue in many situations, usually when virtual mixers refuse to route signals in the way you want.
As far as I can tell, it is totally transparent sound-wise.