If you're talking about identifying non-powered, disconnected wires you can actually see and touch, not tracing them inside walls, get a 2.4 volt incandescent flashlight bulb and solder clip leads to the shell and bottom contact. (Or use an appropriate lamp holder, if you want to spend more and/or not solder.) Also get a holder for 2 D cells or 2 AA cells, and solder or otherwise clip leads to the + and - contacts.

To use it, start at one end of the wire. Disconnect it from the breaker, receptacle, wirenut, or whatever. Clip the battery holder to ground, and to the wire you want to trace (polarity doesn't matter). Now go to the test site. Clip one clip lead from the bulb to ground, and probe the mystery wires with the other clip lead until the bulb lights up. There you go.

If the light is too hard to see, you can also use a 3v buzzer or Sonalert type gadget, if the noise won't drive you buggy. Don't use an LED though, it will light up too readily with high resistance in series.

David, I can't believe I didn't think about that. Now I feel a bit silly. Unpowered usually will not be an issue since I can flip circuit breakers. Sounds like a great idea, thanks for the tip.

Unless the circuit is LIVE, I use a 77 tone generator and a 200 tracer, these are the standard of the telco guys. If the circuit is live, i use an Ideal circuit breaker finder, though it's usually more reliable to kill the circuit and use the 77/200.