some of the old houses I deal with, use a small(16 gauge?) bare copper wire that runs throughout the house, "connected" to the boxes by wrapping several turns around a nail that attaches the box to the stud
I've seen that too, usually for Kitchens. Never seen Copper though, the ones I've seen seem to be Aluminum. They sometimes go from each device box up to the attic and are twisted together, or joined by some other questionable means. Many times these wires are cut or broken and not repaired simply because no one knows what they are.
I had some pictures around, but can't seem to locate them right now.