Trapdoor

A trapdoor (also known as hatch) is a block used as a horizontal door that fills one block space. It is operated by simply clicking on the trapdoor, or with a redstone signal. They can only be placed on the side of a solid block. When activated, they will open by flipping towards the block they are attached to. If that block is destroyed, the trapdoor will break off. When placed outside, rainfall, water, lava, and snowfall do not pass through a trapdoor, but light is allowed to shine through.

Usage
As with doors, trapdoors can be controlled with redstone circuitry. Redstone in its charged state will cause the trapdoor to open. Conversely, redstone in its uncharged state will close the trapdoor. Trapdoors can be used as a less unsightly method of restraining boats at a dock than doors. It's also possible to create a distributor with a vertical succession of trapdoor linked with redstone. They can also be used to create gates, since they stop horizontal movement as well. Similarly, they can be used to create a gate for a counter-top (such as can be found in a bar or restaurant).

Bugs

 * Trapdoors block movement when approaching from another block on the same level and the trapdoor has a block exactly 2 spaces above it, but it does not block movement in the same case if approaching from another trapdoor, or if you are approaching the trapdoor from a space that is also 2 blocks high. This allows one way drawbridges.
 * When the block that a trapdoor is attached to is powered by a repeater, the trapdoor will only work if the block it is attached to also has redstone or a redstone torch attached to it (the same applies to doors as well).
 * If a trapdoor is placed diagonally to another trapdoor, you will sometimes fall through if walked over.
 * When holding shift, when you get on top of a trapdoor open it then jump out and close it, then walk on the trapdoor again you will fall through.
 * If you have two blocks, one on top of the other, underneath the trapdoor, and you pick away at the top one, when you fall through the trapdoor will close, not doing any damage to you, even though you're only halfway.
 * Even though this isn't technically a bug, when a trapdoor is placed, it will be half way down the block it is placed on, thus making the trapdoor block the top part of the ladder.

Trivia

 * Sand and Gravel, if dropped onto a trapdoor will break into items (similar to how sand or gravel falling on a torch or cart track will do the same).
 * Like doors, trapdoors are not flammable and do not burn.
 * When attached above a block of lava, the trapdoor will appear to catch fire and set adjacent blocks on fire. This makes trapdoors unsuitable as disposal lids unless surrounded by flame-proof blocks. However, the trapdoor will not be destroyed from fire damage, even stimulating fire generation where it normally doesn't appear naturally.
 * Trapdoors will not prevent you from accessing the contents of a chest if the trapdoor is in the closed state directly above it. This means that they are just like any other door and allow you to interact with objects on the other side even if the trapdoor is closed.
 * If you put a trapdoor on a lava block, the lava will disappear and the trapdoor will not be burned. This allows the creation of paths.
 * Trapdoors can be used as ladders by placing them one above each other down a hole. Then hit them open to go down, and hit them closed to go back up.
 * The trapdoor as a dropped resource used to be much bigger than other resources. This has been fixed in Beta 1.8.
 * Trapdoors can be used as fuel in a furnace but burn only as much as a wooden plank.
 * Trapdoors can be pushed by pistons if there's another block for it to connect to or if the block it is currently connected to is pushed with it.
 * Although zombies are able to destroy wooden doors (As of Minecraft 1.2 release), they can't destroy trapdoors.
 * If you remotely open a trapdoor while a mob is standing over it, it will fall through the door. This makes for an effective trap in front of a vulnerable entrance.
 * A trapdoor can muffle the sound of weather.
 * Trapdoors do not block redstone signals going up under them.
 * Two trapdoors placed within a 1x2 doorway/opening, in the open position, makes for a very narrow entry.
 * A trapdoor is effective for building underwater, since water does not pass through it.