I discovered the Devolo Powerline adapters (made in Germany) some years ago. They basically allow you to expand your data network on your house or office with almost nothing but plugging the devices to wall electrical outlets.
How it works is very simple, you plug a Powerline to a wall power outlet and connect your ethernet cable to the computer you would like to connect. Then connect another Powerline near the switch or router of your house and plug an ethernet cable from the the router to this second adapter. The devices will connect to each other using your building powerline cables and will establish a network connection. This technology is called: Power Line Communication or PLC, you can read more about it here.