A small AC Dimmer or Relay module, controllable via Zigbee or another RF protocol. This block can get it's power from the incoming AC line, and can either use SCR dimming to dim the output or a relay to simply turn on or off the power.
You could walk into a room, plug in a few of these modules into the lightbulbs and lamps, and have an instant dance party! The lights could be dimmed like theatrical or concert lighting figures to correspond to the music, or just to set the correct mood for the evening...