Pakratt's Valley Trap
Pakratt0013's Valley Trap was a massive mob grinder built in the ocean on the Season 3 MindCrack server. It lay within the vicinity of Etho's base, which existed in a swamp biome enclosed within the aforementioned ocean. Although a Valley Trap often refers to the specific building on the MindCrack server, it is also a novel mob grinding design. 
Valley Traps are comprised of a shallow trench with a three-wide gap in the middle. One the lowest level of each side of the trench, pressure plates are added to make sure that water from the top of each side of the trench flows in a straight line, maximizing spawning space. Pakratt estimated in a tutorial video that each trench adds an extra 10-20% of open spawning space on top of a level of a normal mob grinder level of comparable size. Although it adds greater spawning area, Pakratt admitted two major disadvantages: It is difficult to turn off once activated and, if made too long, will efficiently push mobs to the center.
The Valley Trap was notably the source for all the gunpowder required by Zisteau to completely detonate Desert Springs. Zisteau farmed gunpowder for a week before he was able to acquire all the TNT he needed to complete the apocalyptic event on his village.