Here is a cool system. A knock on the door rings a remote door bell! I was thinking about doing something similar some time ago. My idea was to do it in reverse though, when someone rings the doorbell I would make a solenoid knock on the door. I think this would provide some entertainment for the delivery people. 
“No Doorbell? No Problem! Knock-Knock Door Chime automatically alerts you when someone is knocking at your door.
How It Works:
* Wireless system allows you to place the lightweight chime anywhere in your home or office, up to 100 feet away.
* Intelligent sensor is only activated by a knock-knock sound.
* Easy installation and no unsightly doorbell button. Simply mount the sensor on the inside of your door with double-sided tape.”
Via: Gizmodo
