House wiring and grounding. How do you retrofit an old house and ground it?
It's perfectly legal to run JUST a ground wire to retrofit old work.   You do not need to also pull all the conductors.  This is legal as of NEC 2014, so if your region hasn't adopted it yet, just wait.  

People who say "might as well pull all new conductors" do not fully understand what the new rule permits.  Retrofit grounds do not need to follow the same path as the conductors.  What's more, you can borrow/share grounds from one circuit to another as long as they all terminate back at the same panel, and are of large enough size.  That is much easier than pulling all new homeruns!  For instance you can run a 10 AWG ground to a clothes dryer, and any nearby 20A outlet can simply ground to that, etc. etc.  

It is also both legal and safe to put GFCI protection on ungrounded receptacles or circuits.  GFCI protection is safer than a ground, although not as awesome for surge suppressors and radios.