There's a key item used only in The Subspace Emissary in Super Smash Bros. Brawl. It's used to unlock doors, but can also be thrown and used to attack. If dropped, however, it will return to it's original starting place.
In The Subspace Emissary, there are often locked doors. This key is the item you need to unlock these doors. You can touch a door while holding the key, or you can even throw the key at a door to unlock it. If you lose the key, it will return to the place you originally found it. This item is vital to your progress, so do your best not to lose it.