Although it's been reported that this door chain design might just be an April's Fools day, I fell for it and I think it is just awesome! As long as you do not mind spending a little more time going through a maze to get to the other end than you would on a normal door chain, ofcourse.
The Defendius door chain is designed by Art Lebedev studios who are famous for their creative designs.
The chain is long enough to reach the far end of the maze. And because there are many places throughout the maze that you can get stuck, you can actually use that to control how big of a gap you want the door to have if you do not want to have someone stick his hand in.
After a few uses, I am sure it will be very easy to use as you will remember the path through the maze, but it will be great to watch your guests puzzled by how to open the damn door!