Slimes

The slime is a translucent, gelatinous, green mob varying in size, health and attack strength. It is also rather rare, and since introduction has been removed and re-added several times.

Their only drop is slime balls, a necessary component for sticky pistons and certain potions. They can only be found in the bottom 40 layers of a map, meaning if you want access to their drops you will have to find a cave system very far down. I would recommend level 20 as the preferred level. Upon death or seeing the player the slime will do several things. All of the slimes will, upon sight, move toward and attempt to attack the player. But the small ones can deal no damage. Upon death, the larger slimes will drop nothing, but instead break up into smaller slimes. These smaller slimes drop slime balls.

A disclaimer when slime ball hunting, do not use swords. Killing for example a medium slime, may do so much damage the small slime that drop the slime balls can't spawn, netting you zero slime balls. I would recommend possibly using a stone sword on the biggest level, but after that use your fists and back up to avoid being hit so you don't overkill the slime.

Squid

The squid was made in Beta 1.2, serving as a passive mob and the only dropper of ink sacs, a component necessary to make black wool. They will only spawn in water and prior to update 1.8 they were used as pets in man-made aquariums. The squid has no way of attacking the player, though due to being underwater the player may drown if not careful.