Internet via the power line

The last hotel I was in offered wired internet to all the rooms. The real cool thing was that they didn't do this through a cable or DSL modem. Instead, the internet signal traveled via the power lines, so that the modem was simply plugged into the wall socket:

Over cocktails, I tried to explain the internet delivery mechanism to another guest at the hotel but she was not a tech-nerd and couldn't grok how cool this was.