The Semiautomatic Nether Wart farm is a handy farm to have. It allows you to collect Nether Wart, a brewing ingredient, without walking around, slowed by Soul Sand, wearily collecting the plants. A modified version of this can be used in a Cocoa Bean farm or Carrot/Potato/Wheat/Pumpkin Seed/Melon Seed farm.
Make a row of Soul Sand less than 8 blocks long. Then plant the Nether Wart. Place fences on either side of the row.
Place a Dispenser at one end of the Soul Sand. Then put a water bucket in it. A means of activation (a lever works best, but Redstone Torches and Redstone Blocks also work) is also required. However, if you are redstone smart, you could set up a timer set to just over the length of time taken for Nether wart to grow.
Put a Hopper linking to a Chest at the opposite end of the Soul Sand row. It should be at the same level as the sand.
How it works
When the dispenser activates, a wave of water uproots the Nether Wart and sends it rushing toward the hopper. The Hopper picks it up and puts it in the chest. You can open the chest and replant the Nether Wart.