Deployer

Deployers use items in much the same way a player does.

A Deployer has nine slots in its inventory. (You can feed it with tubes). Upon receiving a redstone pulse, it examines its contents in order (left-to-right, top-to-bottom), and when it finds a non-empty slot, it emulates a player's right-click action when holding this item. It can place blocks, fill buckets, empty buckets, till dirt, make fire, and do almost everything that a player can do—except for breaking blocks, which is what a Block Breaker does. Note: for a deployer to place a bucket of a liquid the deplyer must be set up so it deploys into an area where the liquid will not flow anywhere. Ex: a 1x1x1 area

Uses
Deployers do not seem to be able to scoop liquids into buckets directly from source blocks, but they can scoop out of Tanks, so setting up a Pump first is needed if they are to be handling liquids. If using Transposer to remove filled buckets from the Deployer, make sure you set it on a longer Timer than the one sending redstone pulses to the Deployer, otherwise you will take empty buckets instead of full ones.

You can also set it to drop Sand or Gravel down a hole or swing a weapon as some sort of defensive structure.

Another interesting feature of the Deployer is, since it simulates the player placing blocks, it can be used to create Snow and Iron Golems, which couldn't be created using pistons.

It is also possible to upgrade the deployer to an Assemblerfor more complex uses.