This is an easy and quick to build in survival mode mob grinder and xp farm. All you need to do is find a dungeon (zombie or skeleton, doesn't matter) and make the water elevator as shown in the screenshots. By natural instinct the mobs will jump up in water, traveling upwards in the elevator. Once at the top, the water flow pushes them down a tunnel and into a shaft. They fall down the shaft (around 25 blocks) and lose most of there health. They gather up in the 1x1 area and you can kill them off with 1 punch (sometimes 2) and get their xp and items. You can alter the effect by lowering or raising the shaft so they die on impact or have less or more health remaining, depending on if you want just mob drops or quick xp. This is a quick way to get lots of xp and items such as bones, arrows, and even enchanted bows (check the second to last screenshot, 1 skeleton has an enchanted bow on him)

Enjoy!