How about a good dog that barks?
Guy I used to work with was robbed during the day when no one was home, and the robbers distracted their dog by opening up the fridge and pulling out food. When they got home, the dog was fat and happy, and they were cleaned out of everything valuable and small enough to carry away in a backpack. They figure the dog must have barked like crazy at first, but the neighbors didn't hear or pay attention before the dog was distracted.
A really, really loud obnoxious alarm system seems to me to be the best deterrent along these lines.