You know, I've never thought about a separate system. I usually have a cell phone at the barn/shop but, as Glenn says, what if things go REALLY bad and can't get the cell to work, etc.
Air horn maybe? If so, where's the best place to hang it? I live pretty far out in the country (don't get Monday Night Football until Thursday night), so if the wife's not here, who would hear an air horn?
Be interesting to read what solutions others come up with.