This is a red stone contraption that was really fun to make. What it does is it spawns pigs, kills them and harvests their drops.

How?
The pigs spawn on the grassy areas and wander in to the short water current dropping them into another current that pushes them into the 'waiting area'.
When the lever is pulled it pushes the pigs into the 'killing chamber'!
You can choose it to automatically shoot the arrows or you can do it manually by repeatedly pulling the lever. When all the pigs in the killing chamber are killed, you pull the water release lever that opens a small hatch in the killing chamber and the water flushes out all the drops.

I am thinking of making a variation that you can collect experience orbs from because currently in 1.8 when a dispenser kills a mob, no orbs are dropped.