If you're pretty handy with some DIY electronics (which I think you might be), you could consider the MonaLisa board (previously known as the Thingshield) at this thread:
I think the Thingshield was originally designed as a shield for Arduinos etc, but I know that the MonaLisa board doesn't actually need to be connected to any other board to work. I think it can work as a Zigbee relay switch out of the box. A quote from the user manual: "For instance, sending the command "on." from the hub to the board will tell it to turn on all its digital outputs."
