X10 manufactures home automation products that are fairly cheap, easy to setup & use, and very handy & useful.  These products allow you to control lights & appliances from the touch of a button on a remote control. A radio frequency (RF) signal is used by the remote controls so no direct line of sight is needed. When using an infrared (IR) remote, like most TV remotes, you must point the remote directly at the device to be controlled.  With X10, the remote control sends a signal to a transmitting device that is plugged into a standard 110v house outlet.  This transceiver will then send a signal through the electrical wiring to control a specific device.

A computer can also act as your remote control if an interface, such as the firecracker, is used. This device plugs into a standard 9-pin serial port. X10 provides software that allows you to control your X10 devices as you would from a remote control.  This is nice, but it gets much more interesting if you have a linux machine on your home network. If you plug the firecracker into a linux machine you can control your X10 devices from the linux machine... and also from anywhere you have access to this linux machine.

I have a linux server in my house that is accessible from the internet.  Just yesterday I plugged in my firecracker to this Ubuntu linux server.  After installing the bottlerocket program (apt-get install bottlerocket), I was quickly controlling my X10 devices from a terminal session connected to the server.  And taking this one step further, using a PHP program placed on the webserver running on this machine - I can now control my X10 devices from a webpage that is accessible from the internet.

X10 links: 

http://www.x10.com/ 

http://www.x10.com/automation/firecracker.htm 

Bottlerocket:

PHP-BottleRocket/SOSDG

http://www.sosdg.org/sites/sosdg.org/files/x10.php_1.txt