With a $99 Siemens SpeedStream Powerline Wi-Fi gateway and a regular powerline gateway, you can plug your DSL modem or gateway into a wall-socket, and then connect the Powerline WiFi gateway into any other socket and provide Internet service to it. So, if you want to put a WiFi access-point on your roof for your neighbors but your DSL modem is in your living room on the first floor, you can spare yourself the hassle of running an Ethernet wire up the staircases with one of these.
Link to Glenn Fleishmann's summary of the story