With X-10 devices, you can turn lights and appliances on and off, set up a security system and control it all from your computer. You can set up macros to do multiple operations triggered by a single event. You can even use voice commands to tell your devices how to respond, or trigger events by light or motion detection. X-10 is typically marketed as home automation equipment, but it's also useful in the office. For instance, an X-10 motion detector set up in the computer room could trigger emergency lights and a call to your phone or pager using a voice dialer device.
Some devices do both.
Receivers respond by turning attached appliances on or off, dimming lights or completing other tasks. Transmitters send signals over normal home wiring at the push of a button or when triggered by light, motion or voice command. There is a wide variety of X-10 devices available, and there are many vendors for each type. You can buy a basic starter kit such as X-10 Corp.'s ActiveHome or IBM's Home Director, but the best control system I've seen is Home Control Assistant from Advanced Quonset Technology. It lets you display all your X-10 devices on a floor plan of your home, while a separate pane lets you zero in on a specific floor, device or controller.