Pumpkin

Pumpkin is a type of block which was introduced in the Halloween update. They are close together, appearing on grass in a similar manner to that of mushrooms.

Currently, you can only find them as pre-carved Jack-O-Lanterns. It is unknown if this is a glitch or not. As originally planned, you were supposed to find them as normal Pumpkins and carve them yourself.

Harvested pumpkin can be equipped as a helmet without any actual armor value; it is, however, unique due to it limiting the player's view of the world as if inside the pumpkin.

Their spawn algorithm is unknown as of now. You can create a Jack-O-Lantern by putting said pumpkin on a crafting table and  putting a torch under it. When placed, it emits a light that extends somewhat further to that of a torch. The only flaw is that you cannot wear it or change it back to its original form.

Pumpkins cannot be placed without a block under it. However, much like the fence, you can place it, then remove the block directly under it. It won't fall, it also reveals that the bottom texture is the same as the top. Follow the link below to view a video of Minecraft pumpkins.

Video of Pumpkins in Action