Pressure Plate

A Pressure Plate is a type of switch that can be placed onto a surface of a block. If connected by wire to an object (or placed adjacent to a block that can be powered), they will provide power to that object or block when stepped on by either a player or a mob (as well as other items; see below), and will stop when stepped off. Pressure plates are one of the seven types of switches currently in the game.

The Weighted Pressure Plate, a variant of the standard pressure plate, outputs a redstone signal dependant on the amount of items on it.

Crafting
There are currently only two types of pressure plates: wooden and stone. Wooden pressure plates are made from 2 Wood Planks placed side by side, and stone pressure plates are made with stone (not cobblestone) in the same fashion.

Triggering
Pressure plates may be triggered in the following ways:
 * A player or mob standing on a plate.
 * A player flying in close proximity on top of a plate.
 * A dropped item (the kind you can pick up by walking onto) on a wooden plate (stone plates are not affected). Sand or gravel falling onto a pressure plate will convert into a dropped block, but it will not necessarily land on the pressure plate unless there are walls around it.
 * Minecarts:
 * Any type of minecart will trigger a wooden plate.
 * A minecart with a mob or player riding it will trigger a stone plate (however, storage minecarts and powered minecarts do not trigger stone plates).
 * Pressure plates do not act as rails, so the cart will stop on the plate unless it has very high momentum.
 * A minecart on a curved track triggers a pressure plate adjacent to the track (think about the cart's corners sticking out) if there are two or more successive curves. (Two curves are needed to get the cart on a diagonal path and thus the edge sticking out over the track. If a single curve is laid down the cart just changes by a 90 degree angle.)
 * Fishing rods
 * A fishing rod can activate a wooden pressure plate (seen in this video)
 * Arrows (wood pressure plates only):
 * A plate enabled with this method remains turned on until the arrow is recollected or despawns (an arrow takes one minute to disappear).

Pressure Plates are often used in traps. For example, you can wire a pressure plate to TNT, to make it explode and damage the victim and the surrounding area (can be prevented with water, which only damages the victim, not blocks). An example of a naturally occurring TNT trap occurs in desert temples (as of 12w21a).Another trap is to create a plus in the ground with pistons. Put a pressure plate in the middle of the plus on a block of dirt and put a block above the middle [stack 3 blocks and take off 2 starting from bottom]. Keep it up during the night and lure any mob [excepting the facts that endermen are 3 blocks tall and that spiders and cave spiders can climb out of the trap] into it, trapping the mob with an open 1×1 space to hit the mob.

Pressure Plates can also be used to make sure that you do not forget to close the door to your home. Just place a pressure plate just inside of the door.

Note blocks
With the release of note blocks players can make songs using the pressure plates to play a note. Players can remove the pressing sound by modifying the plates' sound file in the .minecraft folder. Players can replace the press sound with silence via the aid of sound editing software such as Audacity, then set the file as read-only, thus preventing Minecraft from re-downloading the sound file. This enables a screen recording of the created song without having the pressure plate sound messing up the recording. The sound file is located at .minecraft\resources\newsound\random\click.ogg. Note: this is also the sound for the in-game menu buttons.

Trivia

 * If you try to place cake on a pressure plate, the cake will float as the pressure plate technically takes up the entire block it is located in.
 * When a pressure plate is on a normal block, it will power objects on the same level as the block. If a pressure plate is on a fence, the plate will only power objects at the same level as the pressure plate and it will not transfer power through another block to redstone wire.
 * Pressure plates activate when you stand on them if you're within 0.25 blocks of the ground. This means you can fly close to the ground and still activate them when flying.