Hidden doors can even be functional. This folding bookcase door is perfect for a book lover who wants to make the most of a home's square footage. Why waste potential storage space with a plain old door in it when you can have a door and bookcase in one?

When this door is closed, you've got two columns of shelves for your books or knickknacks, and when you slide the bookcases open, you can stroll through to a secret passage, hidden vault, or the messy bedroom you just didn't want your guests to chance upon.

From Barn Door Hardware