Soul Sand

Slow sand (aka "mud", "hell mud" or "soul sand") is a brown-grey dirt-type block, found in The Nether. This material has the most of same properties as Dirt except it lacks grass. It has two unique features. It can slow the movement of any player or NPC, and players sink slightly into the block if walked on. It also proves to be an effective building material when creating a dock. Boats will not be destroyed when colliding with a slow sand block at high speeds. It is not flammable. Also, sneaking on a slow sand block will not prevent you from walking off of the block.

Trivia

 * The texture resembles a collage of faces. These faces are said to look like those of Creepers.
 * This block stopped generating in the v1.2.0_01 update; in its place, gravel was generated. This was fixed in the v1.2.0_02 update.
 * The first appearance of this block was on Notch's blog.