Slime Block

The Slime Block is an upcoming, transparent block that resembles a Slime. If the player (or any other mob) jumps onto the block, they bounce on it at a slightly lower height than their original jump. Then when they land back on the block, they bounce at an even lower height than the previous bounce, and the cycle repeats until the player is just standing on the block. The block will be introduced in the Minecraft 1.8 version for PC. Such blocks are now able to be implemented due to a new rendering system. Slime blocks also causes players to move slower while walking on it. Their speed is comparable to Sneaking, and is slower than walking on Soul Sand. Both players and mobs who fall onto this block will take no fall damage.

As a crafting ingredient
A Slime Block can be crafted back into 9 Slimeballs, making it useful as a storage block.

Behaviour
Slime Blocks will cause Players and Mobs to bounce at a height relative to the height at which the player/mob fell at. Items, falling sand/gravel etc. will not be affected by Slime Blocks.

Slime Blocks are the only block that can be broken at speeds quicker than creative in survival without the aid of Haste II from a beacon.

Trivia

 * Before Snapshot 14w02b, Slime Blocks where known as "Bouncy Blocks" as their real names had not been announced yet.