Item Frame

An Item Frame is an item that displays the item or block that is inside it.

Obtaining
An empty item frame can be broken simply by punching it. If the item frame contains an item, punching it will drop the item and leave the frame on the wall.

Because item frames are entities, they can be broken in Adventure mode.

Usage
Item frames can be placed on cactus blocks, pressure plates, fences, trees, slabs, cobblestone walls, chests, doors, and all solid blocks. As item frames are an entity, multiple item frames can occupy the same block, although on different faces. It also allows non-solid blocks to occupy the same space.

Players can place items in the frame by while holding the item. If a map is placed in an item frame, it will enlarge to show the map in the size of a full block. If the item in the item frame has been renamed using an anvil, it will display its custom name when the cursor is over the item frame. If the player places a block inside the frame, it is displayed in miniature half in, half out of the block. This can be most notably seen on stairs or slabs.

Rotation
The displayed item can then be rotated by right-clicking the frame. Items displayed in the item frame can turn 45° diagonally, with a total of 8 possibilities, and the frame will output a redstone signal depending on the rotation phase which can be interpreted by a redstone comparator. As long as the frame remains placed, it remembers the orientation of the last item it held, and uses it for the next item it holds.

Trivia

 * The item frame is based upon a suggestion on reddit, like the ender chest and flower pot.
 * Since you can't use commands to enchant in the console editions of Minecraft, item frames are an alternate way to copy enchanted items for custom maps.