Slime

The slime is the fifth hostile mob, added in Minecraft Alpha 1.0.11. They appear as green semi-transparent cubes. Slimes come in four official sizes: huge, big, normal, and small. When a player attacks a huge, big or normal-sized slime, it will break off into four smaller sized slimes (huge to big, big to normal, normal to small) unless a sword is used. Slimes will not grow larger over time. Slimes of any size will burn instantly upon touching lava and die moments later without splitting into smaller slimes. Only huge, big and normal-size slimes can deal damage; small ones are harmless but can still push you around.

Slimes are found in deep caves, usually in large, cavernous spaces (making them appear often in large, open mines). This is partially caused by the fact that they will not spawn more than 15 squares above the bottom of the map (10 squares  before the topmost bedrock). They are currently available in all difficulties, even in peaceful where small slimes will spawn. In addition, a bug causes slimes of any size to not be removed when switching to peaceful. The slime is the first mob that can spawn in both light and dark areas.

By editing the mob's files, it is possible to make the slimes much bigger, some values creating 50 block high slimes. Slimes cannot swim, therefore they will drown if they are submerged completely in water, and jumping does not get them out of water. On 10/10/10, Notch added slimes into SMP.

Drops
Alpha:
 * 0-2 units of Slime Balls