Oh and another great way of resource farming is to create a tower that rises high enough into the sky that it will kill any mob falling down the center. Create a hollow middle about two to four blocks long and wide, followed by a large enclosed space at the top. From each edge create a channel through which to create a small river from, with raised platforms in the corners. Place the water so that the end of the stream just reaches the edge of the hole in the center, making sure the water does not fall down the drop. Since this space at the top is enclosed, it will be permanently dark, meaning mobs will always spawn there. They will roam into the water which will push them down the shaft, killing them on the floor below. Come back occasionally to collect things like gunpowder, bones and arrows, the occasional ender pearl, silk and spider eyes, rotten flesh, and if you are really lucky the odd slime ball. Be careful though, spiders sometimes latch onto the side and can survive the drop, plus creepers can explode on the way down if you are at the bottom of the shaft.
I had fun once, it was awful.