Instead of attaching things to an Arduino to expand the input and output capabilities of your network, you could also consider off-the-shelf ZigBee devices. In particular, devices that implement the ZigBee Home Automation specification (covered in more detail next) can typically add useful things, such as on/off control of power outlets, (wall) switches or dials, displays, various sensors, and so on.
In this section, you will see how to remotely control a power outlet that supports the ZigBee Home Automation standard, making it turn on and off. Since the protocols and specifications needed for this are complex and big, this will only cover the most basic use case. Implementing the complete ZigBee Home Automation specification using an Arduino and XBee module could probably fill a book on its own, but this section should give you enough info to allow basic control of an on/off device and give you a starting point to implement other things, as well.
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