From BlenderWiki

Symmetrize

The symmetrize operation makes the input mesh elements symmetrical, but unlike mirroring it only copies in one direction. The edges and faces that cross the plane of symmetry are split as needed to enforce symmetry.

Usage

The symmetrize operator is only available in edit mode. It is accessible from the Mesh menu in the 3D View's header and from the Specials menu (WKEY). Select a group of vertices, edges, and/or faces and run the operator. The operator has a "Direction" property that controls the plane of symmetry and whether to flip from positive to negative or negative to positive.

This example shows the effect of changing the direction property. The original input mesh is at left. In the middle, it was symmetrized from negative X to positive X. At right, it was symmetrized from positive X to negative X.

Code

The code can currently be accessed from the symmetrize branch of this git repository.