You'll probably need relays to do the actual switching, and you are likely to need driver transistors / MOSFETs to drive the relays from the Arduino. It might be a good idea to use optoisolators between the Arduino and high power stuff, in which case you'll need a separate isolated power supply to run the low voltage section of the high power side. If you do have optoisolation, then you might want to consider solid state switching with triacs or thyristors, in which case do you want to do zero crossing switching of the AC to minimise electrical interference?
It all depends rather a lot on the actual load you want to work with. Voltage? Current? Power? Reactive or resistive load?
Do you want to build a solid reliable system or a quick and dirty lash up? Remember the traditional rule of engineering - you can have quick, cheap or good. If your lucky, and skillful, you might get 2 out of 3.