Pumpkin

Pumpkins are blocks that appear on grass, spawned in random clusters in most Overworld biomes, though somewhat rarely.

Obtaining
Pumpkins can be mined with any tool, but axes are the quickest.

Natural generation
Pumpkins may generate naturally in most biomes in the Overworld.

Farming
Pumpkin farming strategies parallel those of melons. Planted pumpkin seeds grow a central stem that, after maturing, spawns pumpkins randomly in adjacent vacant dirt/grass/farmland. If a pumpkin is harvested without also cutting the central stalk, another pumpkin will spawn afterwards without requiring replanting or waiting for the stalk to mature again.

Pumpkin stems take around 10 to 30 minutes to fully develop. Bone Meal can be used in pumpkin stems to become fully-grown, but will not produce a pumpkin immediately.

Helmet
A pumpkin can be equipped as a helmet without any actual armor value, but will instead function as a mask that allows the player to look at Endermen without provoking an attack. When worn, they limit the player's viewing area to a mask pattern that resembles the pumpkin's carved face.

Building Golems
Pumpkins 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 pumpkin must be placed last or the Golem will not spawn.

Placement
Pumpkins cannot be placed without a block under them, but once placed, will remain even if the block underneath is mined away. When placed, pumpkins automatically face the player. When a pumpkin is pushed by a piston, it breaks. Pumpkins also cannot be placed on a transparent block.

Trivia

 * The pumpkin is the only headgear that obstructs the player's view.
 * Regardless of the direction the pumpkin is facing, the upper texture is always oriented the same way.
 * Undead mobs (except Withers) and Spider Jockeys have a chance to spawn wearing a Pumpkin (or, more rarely, a Jack o'Lantern) on October 31 (Halloween). While Zombies and Skeletons burn normally in daylight, they do not do so when wearing a helmet, and these do count.  Note that the mobs will not drop it if killed.
 * It was originally stated by Notch that they were to be found and then subsequently carved by crafting. Jeb has stated that pumpkins may eventually spawn uncarved.