Head

Mob heads are decorative blocks. There are five variants of mob heads: Skeleton, Wither Skeleton, Zombie, The Player and Creeper. The Wither Skeleton variant is a rare drop from the respective mob, with the other heads only available in Creative mode.

Obtaining
The Wither Skeleton skull is a rare drop of Wither Skeletons, while the other heads are only available in Creative mode. Obtaining a human head with the same texture as a specific player's head requires /give, and is achieved by changing the NBT tag of SkullOwner to the player's username.

Usage
The main use of heads is for decoration. They can be oriented in 16 different directions on top of a block, and 4 directions on the sides of blocks, similar to signs. They cannot be placed on top of each other.

The player can also wear heads as disguise, similarly to pumpkins. This will only replace the first layer of the player's skin. If the player has a second layer over their head, it will still render over the head (this also applies to Zombie Pigmen wearing heads). They can also be used as a decorative disguise, using dyed leather armor. Occasionally, another mob may pick up a skull and wear it just as the player would. This is common when a wither skeleton drops a skull next to another mob in the Nether. The only mobs which can wear the heads are the Skeleton, Wither Skeleton, Zombie, and Zombie Pigmen. Other mobs can only wear heads when spawned using custom spawners via mods or third-party tools.

Heads are not able to be pushed by pistons, as they have tile entities.

Spawning a Wither
Wither skeleton skulls are essential for spawning a Wither. To do so, place 4 soul sand in a vertical T-shape, then place three wither skeleton skulls on top. The orientation of the skulls does not matter.

As a crafting ingredient
Heads are used in the crafting of Firework Stars. Firework rockets crafted with said stars will have an explosion shaped like a Creeper face, regardless of which type of head was used.

Player Heads
Human heads have an extra usage for map makers, they can be given NBT data (as shown here) so that they appear with the skin of any Minecraft account. This means if you know that a specific account has a head which you want to display, you can edit the NBT data to make it appear. The command to give yourself a head, skinned as another player, is "/give @p skull 1 3 {SkullOwner:PlayerName}", where @p is the player, and PlayerName is the name of the account of the head you'd like to use.

Mojang has created a number of accounts with specific skins, so that map makers can use common heads without the risk that someone will change their skin. Those players have names in the format "MHF_MobName", for example "MHF_PigZombie" is the name of a Minecraft user with a Zombie Pigman head. MHF stands for "Marc's Head Format". There are also a few blocks and "bonus" heads, for more variety.

The following names/heads have been made available:

Mobs: Blocks: Bonus:
 * MHF_Blaze
 * MHF_CaveSpider
 * MHF_Chicken
 * MHF_Cow
 * MHF_Enderman
 * MHF_Ghast
 * MHF_Golem
 * MHF_Herobrine
 * MHF_LavaSlime
 * MHF_MushroomCow
 * MHF_Ocelot
 * MHF_Pig
 * MHF_PigZombie
 * MHF_Sheep
 * MHF_Slime
 * MHF_Spider
 * MHF_Squid
 * MHF_Villager
 * MHF_Wither
 * MHF_Cactus
 * MHF_Cake
 * MHF_Chest
 * MHF_Melon
 * MHF_OakLog
 * MHF_Pumpkin
 * MHF_TNT
 * MHF_TNT2
 * MHF_Apple
 * MHF_ArrowUp
 * MHF_ArrowDown
 * MHF_ArrowLeft
 * MHF_ArrowRight
 * MHF_Exclamation
 * MHF_Question

†Marc admitted he was not aware of mojang creating these heads.

Trivia

 * When breaking any head, the particle effect of breaking it is the same as soul sand.
 * The most possible reason is because they don't have their own texture, so a texture that made sense was chosen.
 * All heads are the same size (even though Wither Skeletons have larger heads than the other represented mobs).
 * Heads are one of the few blocks that do not have their own textures and take their textures from other objects.
 * Heads will break upon contact with water and lava.
 * The "hat layer" is not represented on mob heads.
 * In demo mode after the demo period has ended, breaking a Wither Skeleton skull will cause it to reappear as a normal skeleton skull.
 * Although mob heads do not protect the player, an enchanted mob head, when worn, will apply the said effects to the player. The only advantage over an identically enchanted helmet is the mob head has no durability rating and thus effectively has infinite durability. The squid mask with Respiration III and Aqua Affinity I could be a scuba mask that never runs out for example.
 * Heads won't cause Cacti to break when placed next to them.