Soul Sand

Slow sand (aka "mud", "hell mud") is a brown-grey dirt type block, found in The Nether. This material has the most of same properties as Dirt except it does not have grass and slows movement of any player or NPC's speed. They also sink slightly into the block if they walk on it.

Trivia
The texture resembles a collage of faces. These faces are said to look like those of Creepers.

This block no longer generated in the v1.2.0_01 update. It still existed in areas in the Nether where it already had generated, but in further areas, it was replaced with gravel. In the v1.2.0_02 update, Slow Sand was made possible to generate again with no apparent change.

The first appearance of this block was on Notch's blog.