This sketchup plugin was developed to improve modeling speed by allow user to mirror objects using already drawn geometry’s middle/end points as mirror planes.
You’ll be able to specify angle for the mirror plane in reference with the edge your mouse cursor is on. (Usage case scenario: to Flip an object around a corner you will use 45 degree angle)
Also it is possible to use the longest, middle or shortest dimension of a group or component as a mirror plane.

Here are some examples: