Jack o'Lantern

Jack 'o' Lanterns (or Jack-O-Lanterns) are solid blocks that provide light. They emit light in all directions, regardless of where their faces point. They are brighter than torches, and will emit light under water.

On Halloween (October 31st), all undead mobs (Zombies, Zombie Villagers, Zombie Pigmen, Skeletons, Skeletons on Spiders and Wither Skeletons) have a chance of spawning with either a Jack 'o' Lantern or an ordinary pumpkin equipped as headgear. However, the worn Jack 'o' Lanterns do not glow or emit light.

Properties
Jack-o-lanterns share many properties with glowstone and beacons, with a luminance of 15 (as compared to torches at 14) and the ability to provide light when submerged under water. Of the three, jack-o-lanterns are arguably the easiest to acquire, as their constituent ingredients can be grown (pumpkins, wood) or easily collected in the overworld (coal, wood). Like all non-sunlight light sources, they will cause nearby snow and ice to melt.

As with pumpkins, Jack 'o' Lanterns can be placed anywhere as long as the blocks directly beneath them are solid and opaque (except TNT); However, they will remain in place if the blocks below them are removed after placement. Jack 'o' Lanterns cannot be placed on transparent blocks, including Fences, Glass, Flowers, Mushrooms, Pressure Plates, Sugar Cane, Torches, Redstone Torches, Redstone, Signs, Glowstone, Ice, Pistons, any Stairs, Iron Bars, Slabs or Sticky Pistons.

Attempting to push a Jack 'o' Lantern with a piston will break it, causing it to drop as a resource. In SMP, Jack 'o' Lanterns appear green on a map.

Uses
Jack 'o' lanterns have a higher light level than torches (15 as opposed to 14). Because it is a block, players can replace wall or floor/ground blocks with them for aesthetically pleasing and versatile embedded light sources, which can be especially attractive for lit paths.

Their ability to be submerged in water affords the ability for docks or harbors to be lit from under water for a pleasing and functional effect that is useful to players in boats that need to see where to stop. They can also be used during underwater exploration, such as in lakes and oceans, to make underwater navigation significantly easier. Jack 'o' lanterns can be used as light sources for growing crops at night, and can be placed under irrigation paths to provide an aesthetic ambient glow for crops.

The jack-o-lantern is one of few directional blocks, having a face image on one side and none on the other three lateral sides. The direction this images faces can be controlled via the direction the player is facing when placing the block: a jack-o-lantern will face toward the player placing it. This can be used as a directional indicator.

Jack 'o' Lanterns are useful as 'breadcrumbs' in The Nether, as unlike torches, they will stay in place even if the netherrack beneath it has been destroyed; and unless it is directly hit, it can often resist Ghast bombs, unlike glowstone. It also stands out more than glowstone, because it cannot be found naturally in The Nether.

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

Trivia

 * Unlike pumpkins, Jack 'o' Lanterns cannot be worn on the player's head.
 * 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.