As a modeller, I need to mirror an object very often, and I am sick of always needing to check the
positive or negative side, axis ...etc. this tool lets me think less about mirroring an object.
Basic idea :
By default, it takes a longer side and mirrors to opposite side. If the object isn't crossed origin plane, then copy and mirror to opposite side.
If any component is selected ( face, edge, vert), then keep selected side and mirror opposite.
Modes:
Origin : use World Origin as centre point
Local : use boundary box of object and measure its centre point.
Pivot : use pivot point as centre, also work if there is rotation to object
Maniputor: with a cut mirror plane where you can adjust it.