A skeleton key! Cool! The skeleton key can be found in the Accessories section of your inventory and looks like a skinny, slightly-deformed skeleton in the shape of a key. Little is known about this item other than that when implemented, it will be a consumable.
How to Obtain
You can loot this item from chests, or trade for it on a market world.
How to Use
As of March 2015, this item's function has not yet been implemented. Rumor has it that it may someday have something to do with unlocking things. People have posited that they will open painting vaults of some kind, but the last official word was that they will someday "be used to unlock special doors, teleporters, and traps." (Said by Mike Laurence on this forum thread in April 2013.)
This item was introduced in April 2013, and has been cluttering player inventories ever since.