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.

Video
Note: As of 1.8, Jack o'Lanterns can no longer be used to create golems.

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.