Jack o'Lantern

Jack 'o' Lanterns (or Jack-O-Lanterns) are solid blocks implemented in the Halloween Update on October 31, 2010. It can be created by combining a pumpkin and a torch. They can be used functionally or decoratively as a light source, and double as a directional indicator.

As with the pumpkin, a Jack 'o' Lantern can be placed anywhere as long as the block directly beneath it is a solid, opaque block (except TNT); thus, they can be placed at the bottom of lakes or oceans, allowing the user to easily explore underwater areas. However, it will remain in place if you remove the block below it after placing it. They emit light in every direction and the face does not need to be visible to emit light, so it can be very useful in creating hidden lights. When placed, a jack-o-lantern will face toward the player placing it.

Jack 'o' Lanterns produce the same amount of light (level 15) and share the same ability to stay lit underwater as glowstone, making them more effective and more reliable than torches. Jack 'o' Lanterns will also melt snow and Ice.

When attempting to push a Jack 'o' Lantern with a piston, it will break and drop as a resource.

Jack 'o' Lanterns can also be used to make a Snow Golem and an Iron Golem, although golems made in this way are no different, aesthetically or functionally, from golems made with regular pumpkins.

Trivia

 * On Halloween (October 31st), all undead (Zombies, Zombie Villagers Zombie Pigmen, Skeletons, Skeletons on Spiders and Wither Skeletons) have a chance of spawning with a Jack 'o' Lantern (or an ordinary pumpkin) equipped as headgear. The worn Jack 'o' Lanterns do not glow or emit light.
 * Unlike pumpkins, Jack 'o' Lanterns cannot be worn on the player's head.
 * Jack 'o' Lanterns cannot be placed on Air, Fences, Glass, Flowers, Mushrooms, Pressure Plates, Sugar Cane, Torches, Redstone Torches, Redstone, Signs, Glowstone, Ice, Pistons, any Stairs, Iron Bars, Slabs or Sticky Pistons.
 * In SMP, Jack 'o' Lanterns appear green on a map.
 * Jack 'o' Lanterns are useful to create cheap breadcrumb beacons in the nether, because unlike torches they will stay in place even if the netherrack it has been placed on is destroyed, and unless it is directly hit it can often resist ghast bombs explosions, unlike glowstone blocks. Also, it stands out more than glowstone beacons because it cannot be found naturally. It is also better than fire, because extended fights against ghasts will often leave the landscape torn apart and burning everywhere.
 * Despite presumably containing a torch as their light source, Jack 'o' Lanterns emit more light than torches.
 * Jack 'o' Lanterns can be used for a slime farm to light the room, as jack 'o' lanterns are opaque blocks, so mobs can spawn on them.