ok... when using ac there's a few options.
a mechanical relay (in this case see my reply to your other post)
you always need the diode in parallel with the relay, any coil that you switch on and off will send you back some nasty voltages that will eventually kill your transistor.
another option is SSR Solid State Relay it's a relay that is built using only solid state components (mostly TRIACs) very nice because they can be connected to arduino straight away, they have an internal optocoupler and there is no mechanical parts (which also means no spark when switching) they are usually more expensive though.
for example see farnell part number 1213176
If you need to dim an AC light there is a very nice Velleman kit that costs something like 12 pounds that can be used to control an incandescent lamp using the PWM out from an arduino. (it's calleds something like voltage controlled dimmer)
you can also build your own SSR if you want to save money, consider that this usually is trouble because of the EMI (interference) generated by not-properly-designed circuits.
for the benefit of global knowledge look at figure 9 at page 6 of this datasheet http://www.farnell.com/datasheets/70624.pdf
the schematic shows a simple SSR where Rin is usually 270 ohm... the optocoupler is a special part MOC3021 that's able to fire triacs. there is also a similar part (MOC30something) that fires only when the ac voltage is crossing the 0 which is quite nice for reduced noise.
I hope this covers the situation a bit.
geez I'm so messy when I write :)