Use the Mirror tool on the Modify shelf tab to mirror copy objects or geometry (depending on whether you use the tool at the Scene or Geometry level).

At the object level, this is useful for rigging one side of a character and then duplicating the bones to the other side. As you duplicate, you can automatically rename nodes, for example r_arm is mirrored as l_arm.

The Mirror Capture tool on the Character shelf tab lets you mirror copy capture attributes (attributes specifying which bones control which parts of the character’s skin) from one side of a character’s skin to the other. See capturing character geometry for more information.