How to flip the view from the camera horizontally? Many Photoshop concept artists flip their canvas horizontally from time to time during painting for a few seconds to better judge the composition. Is it possible in Blender's viewport, using the camera settings maybe?

Of course I could render the image first and then flip it in compositor or any external image editor, but it would be much slower solution than flipping the viewport live.
You can scale the camera negatively along it's local 'x' axis to acheive a flipped view (while looking through the camera).

Select the camera and press S,X,X,-,1

Pressing X twice will scale along the 'local' axis instead of the 'global' axis. 

Alternatively just enter '-1' into the 'x' scale in the object panel: