Drops

Rare drops are items dropped by killed monsters, which occur especially infrequently and in addition to the regular drops. Unlike regular drops, they only occur if the monster is killed by a player, similar to Experience Orbs. All rare drops can also be obtained by other means.

Rare drops have a 2.5% chance of occurring when a monster dies, and a 20% chance of being enchanted rare drops. Using a weapon with the Looting enchantment increases these probabilities. All types of non-enchanted rare drops are equally likely.

History
Rare drops were first added in Minecraft 1.2.

Trivia

 * Rare drops are the only renewable source of Iron.