The Panipulator plugin allows the flipping of panning, flipping of left/right phase individually, and includes a mono button for summing stereo mixes. Not a mix goes by where I don’t use the Panipulator plugin. I consider the ultimate utility plugin.
I have plans of creating specific guides for the Panipulator plugin that show you exactly how to use it effectively. In the mean time, if you have any questions feel feel to post on the Audio Engineering forum.
For more info or to freely download either the Mac or PC VST versions of Panipulator head to the Panipulator page.