Falling Mob Trap

Here is a simple falling trap good for maps like Skyblock. You build a platform above ground about 20 blocks. Create a 4x4 square with the middle 2x2 as a hole. In each direction build out so there is 8 blocks from the hole to the edge of the walls on each side. Fill the rest out in any shape, (square/octagon recommended) and build a roof 2 high, (3 high and endermen will spawn and take away blocks). Cover the roof and have that dark room for mobs to spawn, the water will push them to the hole and they will die from fall damage.

Try it out yourself.