Controlling mains (230 V) supply up to 5/10 amps using Arduino
The PowerSwitch Tail 2 is a great way for experimenters to control mains voltages. It is insulated and internally decoupled, so you only have to hook up 5v, gnd, and a signal line to control it. 

This model supports only 120V, but there are also kits for 230V.



image from sparkfun