Some people will do anything to pack just one more bookcase into a room...
I love books. Chances are, you love books, too. The problem with that is that there is never enough room for all of the books you've read, and never enough for all the ones that you want to get around to reading because physical space is at a premium, and pesky structures like doors get in the way of building bookcases. That is, unless you do what this guy did and build a classic-style "door hidden behind a hinged bookcase". Rather than buy bookcase kits from a furniture store, kenbob@instructables designed his own bookcases and figured out how to mount one of them (they can each hold between 500 and 1000 pounds each, the weight of your average cache of books) to the wall without ripping everything down, or using stabilizing casters on the bottom (which ruins the effect). There's nothing particularly odd behind the hidden door, just the house's crawlspace and a dead storage area, but if you're going to do it, you may as well do it right. The framework constructed is entirely made of steel, so it's much stronger than plain old wooden bookshelves. There are even wooden trim pieces to hide the parts where the bookcase doesn't quite meet the wall.
Total cost? About $350us.
There are images and videos built into this article, so be sure to give everything a close look - he did a very good job, I have to say.