Soul Sand

Soul Sand (also known as Slow Sand) is a brown-gray dirt-type block, only naturally found in The Nether, usually near large bodies of lava in large veins in excess of 100 blocks.

Soul Sand looks somewhat similar to a dirt block, but it has some unique features. It slows the movement of any player or mob, and players sink slightly into the block while walking on it. It is only 7/8 the height of a normal block, so the player cannot walk from a block of Soul Sand to a slab without jumping. Its texture has faces on it similar to tortured souls.

Uses
Soul Sand's slowing effect is much greater when it's placed in water and is useful for making drowning traps more effective against mobs. This block will also prevent water currents from transporting items over it. For example, it is often used in a cannon to stop water from moving the TNT too quickly. When placed directly over an ice block, Soul Sand's slow effect is greatly increased, affecting both mobs and players. The slowing effect can also be used to stop large amounts of mobs at once, making them vulnerable to attacks with a bow.

Soul Sand can be used to suppress the impact from boats when colliding to it at the angle of approximately 6° to 174°, which made it an effective building material when creating a dock.

As of snapshot, Soul Sand is also used to create Withers.

Automatic walkway
Because of its size, Soul Sand can be used in automatic walkways where Soul Sand and a fullsized block are arranged in an alternating pattern and pushed around in a rectangular circuit by pistons. This can be the fastest possible way of transport in Minecraft if it is built properly, however there are several disadvantages:
 * 1) The walkway cannot be more than 16 blocks per span, although multiple spans can be connected.
 * 2) The player must stand at the very edge of the walkway or they will fall through it.
 * Also the end portal frame block, without the eye of ender, can be used to make a one-way walkway (can be obtained in creative mode or with the pick block option); with an Eye of Ender in it has a regular block's height.

One-way and Minecart-only gates


When placed on the ground in front of a two-block high doorway, Soul Sand prevents players from going through it from that direction since it is 1/8 less in size than an actual block. This can be useful for building one-way passages or invisible barriers. This affects mobs too as they have to jump to get out of the slowing effects of the block, thus are unable to get through the door.

However, it is possible to avoid the sand trap by jumping over the Soul Sand and squeezing into the one way door or continuing to jump when trying to go through the entry way. Since you do not sink into the Soul Sand in this case, it is like jumping over an empty space. It is possible for a mob to avoid the trap in this way, but unlikely, since a mob usually only jumps when there is a block one level higher than the mob. For that reason, these kinds of one-way doors will be best served by covering a large (4x9 centered in front of the door itself) rectangle with Soul Sand, such that players and entities will not be able to merely jump over it.

Because rails can be laid on Soul Sand like most other block materials, the slowing ability can be exploited to create minecart-only gates (by placing Soul Sand on both sides of the doorway). This is one of the few ways to allow rails to go directly inside a secure area without risking wandering mobs getting in.

Using the same idea underground, one can create a tunnel trap for wandering mobs and players. By digging out one block of the ceiling of a 1x2 tunnel and placing Soul Sand underneath, players will be unable to easily leave the Soul Sand block without destroying blocks.

Another way to get through the gate is by placing a block above the doorway (see picture). it can be any block above the doorway except air. But there is one problem with this other walkway, it will glitch and you will get stuck in the ground.

I'm not sure at what point in the development of Minecraft these oneway gates stopped working, but at least on my copy of Minecraft 1.4.6 they don't work. You can go through them in either direction.

Growing Nether Wart
Another use for Soul Sand is allowing the player to grow Nether Wart, a primary base ingredient for many varieties of potions. Nether Wart is planted on Soul Sand like a seed, and at full maturity produces multiple Nether Wart when harvested.

Trivia

 * Upon close inspection of this block, the texture resembles a collage of sad faces. This could explain the "soul" in Soul Sand. This feature was also much more noticeable in the Beta Pre-Release versions.
 * The first appearance of this block was on Notch's blog.
 * Soul Sand can prevent zombies and skeletons from burning under certain conditions. If you add glass above their head and prevent them from jumping, they will not ignite.
 * There's a glitch with Soul Sand and sprinting. If you sprint and hit a certain spot, instead of sinking to 7/8 of the block it's possible to sink all the way through it.
 * Because Soul Sand is not a full block, standing near lava while on Soul Sand will set you on fire and make you take lava damage. Be very careful around Soul Sand.
 * The slowest speed you can have while still moving can be achieved by sneaking and blocking with your sword while walking on Soul Sand with ice underneath it through cobwebs and still lava backwards, under the effect of a potion of slowness II.
 * In version 1.2.3, slowly floating into a block of Soul Sand will kill you, making Soul Sand boathouses dangerous.
 * If a water current is pushing you diagonally against a wall and Soul Sand is the block adjacent to your head when the current ends, it will cause you to jump.
 * In the code, soul sand is called "hell sand".