Jack o'Lantern

A jack o'lantern is a solid block that provides light.

Obtaining
Jack o'Lanterns can be mined with any tool, or without a tool, but axes are the quickest.

Light source
Jack o'lanterns have a luminance of 15 and the ability to provide light while submerged under water. Like most light sources, they will melt snow and ice.

Building golems
As of, Jack o'Lanterns can be used to make Snow Golems and Iron Golems as shown below. Snow Golems require snow blocks for their bodies, while Iron Golems require iron blocks. The jack o'lantern must be placed last or the Golem will not spawn.

Trivia

 * On Halloween (October 31), all undead mobs (zombies, zombie villagers, zombie pigmen, skeletons, spider jockies and wither skeletons) have a chance of spawning with either a jack o'lantern or an ordinary pumpkin equipped as headgear. The worn jack o'lanterns do not glow or emit light. Despite the Halloween easter-egg, the player cannot wear a jack o'lantern like they can a pumpkin.
 * In SMP, a jack o'lantern appears green on a map.