Here is a Zombie Pigman farm I created in the nether. 
Originally this was just a pit / trap to lure Pigmen into if they were agro'd and wait for agro cycle to end. 

I had to modify it to auto kill the Pigmen, because it was causing Lag issues on the server I am on. 
The server did not like putting 100+ Pigmen in a small area,

So I redesigned it to kill with Pistons.

You stand on the upper level, and the Pigmen try to get to you, they will go up the 'stairs' and fall onto the pressure plate, which trips the piston, sufficating them.

Drops fall in the hoppers below, and go to a chest for collecting.

This doesn't not work with Baby Pigmen, because they are only one block tall, so I had to make sure there was a place for the Baby's to fall out.

What you see here is a re-creation i built in creative, the Oringlal is built on the Gibcraft server.
 
One of the other players really enjoyed watching the pigmen die.