As we use the X10 protocol to control lights around the house I was quite pleased when I found the X10 library for the Arduino :

X10 Library

This library enables you to send and receive X10 commands from an Arduino module. X10 is a synchronous serial protocol that travels over AC power lines, sending a bit every time the AC power crosses zero volts. It's used in home automation. You can find X10 controllers and devices at http://www.x10.com, http://www.smarthome.com, and more.

This library has been tested using the PL513 one-way X10 controller, and the TW523 two-way X10 controller. Both of these are essentially X10 modems, converting the 5V output of the Arduino into AC signals on the zero crossing.

[From Arduino - X10 ]

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