It is sometimes very handy to be able to control mains electric via an arduino to turn things like lights on and off. [warning]BE VERY CAREFUL hooking up any mains electricity. [/warning]
I built a dual circuit so that two things can be controlled from one board. Here is the fritzing graphic:
Click on the above images to enlarge them and see the parts used.
Input (3 pins) comes from a simple arduino circuit (GND, 5V Power and another pin to control the relay state). A simple arduino sketch can toggle the relay off/on.
Here is the fritzing source so you can modify it yourself.