Soul Sand

Soul Sand (also known as Slow Sand) is a block only naturally found in The Nether.

Obtaining
Soul sand can be mined with any tool, or without a tool, but shovels are the quickest.

Natural generation
Soul sand is found usually near large bodies of lava in large veins in excess of 100 blocks. It is also found naturally in Nether fortresses, in the Nether Wart room.

Usage
Soul sand slows the movement of any player or mob, and players sink slightly into the block while walking on it. Its slowing effect is increased when it's placed in water, or above ice, packed ice, or slime blocks.

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.

Soul sand can be used to suppress the impact from boats when colliding to it at the angle of approximately 6° to 174°.

Growing Nether Wart
Soul sand is used to 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.

Withers
4 blocks of soul sand are used in the construction of a Wither.

Trivia

 * The first appearance of this block was on Notch's blog.
 * 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.
 * 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 "hellsand".
 * If Snow or carpet is over Soul Sand, the effects of it being a partial block are avoided, as you will be walking on the cover block, not the Soul Sand.
 * Because of the ability to reset the Nether in the Xbox 360 edition, Soul Sand is renewable in that edition. Although you can reset the Nether in the PC edition by deleting the DIM-1 folder in a Minecraft world file, some may consider this cheating.