Sponge

Sponge is a block notable for being able to erase water within a 5×5×5 area. However, this function only exists within Minecraft Classic. In Beta the block has no use.

Sponge is only available as a normal building block in Creative mode. However, in Beta it is possible to be obtained via inventory editing. As of Beta, the sponges have no absorbent properties in survival mode. They act as a stone block would, only weaker.

History
Sponge is a type of building block added in the update to 0.0.19a client update on June 20th, 2009.

Properties
Sponge is not affected by gravity, unlike sand, and it will not fall if a block is deleted underneath it. Sponges do not need light to survive, and do not decay (sponges used to decay in the implementation of the 0.0.19a update, but this was changed within 50 minutes of the release)

Building Aid
A sponge can be placed around an area underground clearing the area of water. This can greatly help construction of underwater bases, homes etc. After the player constructed the structure, the sponges can be easily removed.

As an anti-griefing measure
A sponge can be hidden in walls, etc. as a defense against griefers. This can confuse and slow down griefers when attempting to flood an underground development, leaving more time for them to be caught and stopped. In small rooms, this measure, when implemented in every vertex of said space, can stop that room flooding altogether. Note: In Minecraft Beta the block will not remove water and will have no use for anti-griefing purposes.

Schwamm