Normal Modifier
The normal modifier simply flips or joins together the normals of an object.
This is a tool to remember when you have finished working with a complex mesh, like a 3D character, because it is possible some normals may be the wrong way round. This is easily fixed by applying the normal modifier to the offending polygons and then collapsing the stack.

Above, a cube with correct normals and a cube with flipped normals, resulting in the cubes texture normals being inside-out.
