For quite a few of the Ellentriek#11 projects, participants wanted to connect Arduino to the main current system: 220 Volt.
As this can be tricky, it’s good to look for solid solutions.
I have found a relay set-up which is safe to use with 220 volt and arduino (yeay!).

This is the little board:
http://www.sparkfun.com/products/9096
and you need of course to add all the components. The transistor and diode are essential for safety (making sure that the current only goes into one direction).

There is a tutorial on how to build it:
http://www.sparkfun.com/tutorials/119

I buy all my Sparkfun stuff here, which makes sure that you avoid the international shipping costs and the US taxes.

(I got the relay specs here)