Software to horizonally flip (not rotate) OSX's monitor output?
I realize how old this is, but the answer is VLC player. I have an El Capitan Macbook 13". Open VLC, then Window - Video Effects - Geometry - Transform. From there you can rotate 90/180/270, flip horizontally or vertically, magnify, clone etc. Perfect if you have a p/w locked no remote garage sale projector like mine.