Huge Minecraft Mob Trap!
I use this trap to get items for crafting without putting myself in danger of getting hurt by then mobs. It is simple. The mobs spawn, and progressively get pushed into the center of the 100x100 room. Once they fall down a hole in the center, they get killed by lava, and their loot drops get delivered by water. I made this trap by myself, and I did not just download somebody's save file.
Because I'm lazy, I used MCedit to make the large 100x100 opening underground. Then I carved out everything out with a picaxe.