Cow

Cows are passive mobs that are the sole source of leather and milk. They stand 1.6875 blocks tall and are 0.625 blocks wide and 1.5 blocks long.

Uses
Each cow drops 0 - 2 pieces of leather when killed, which can be used to craft a renewable type of weak armor. Cows are an indispensable part of making cake (provided the player has three buckets), as an unlimited supply of milk can be obtained from a cow.

Behavior
Cows roam the Overworld in herds of 4 - 8 and can be observed jumping up slight inclines and falling to their deaths from tall cliffs and overhangs. Cows make no attempt to stay out of water, and will bob up and down to stay afloat. A cow can be milked by using (right-clicking while holding) an empty bucket on its pink udder.

History
Cows were introduced in Alpha 1.0.8, dropping leather when killed and producing milk when the player used a bucket on a cow. This milk had no use until Beta 1.2, when Notch introduced the crafting recipe for cake that required 3 buckets of it.

Future
In the Beta 1.8 'Adventure Update', Cows will be a source of Raw Beef, which will turn into Steak when put in a Furnace. They will also have a proper model for their udders (at 0:02 (pause))

Trivia

 * Cows in the PC Gamer demo of Minecraft are branded with a 'PCG' logo on their side.
 * In SMP, clicking anywhere on the cow with a bucket will fill it with Milk, but only at the maximum distance at which you can use the bucket.
 * As of Beta 1.3_01, the pink of the cow's ear may also be milked.