Mesh Mirror Cut a plugin by Michael Pryor which now is a part of Pufferfish, is inspired by Autodesk Maya’s Mirror Cut tool, which is great for creating symmetry. Mirror cut trims a mesh at a plane then mirrors it over the plane and stitches the original + mirrored meshes together into one new clean mesh.

It is really handy to run in a loop with Anemone for generating many variations of a geometry, essentially creating a sort of library of parts. Also, by compounding multiple iterations of mirror cut onto one geometry you quickly get some complex shapes emerging from simple ones. The component works with opened and closed meshes. Will appear in the Mesh > Util tab.