A German design firm, Jjoo, has created a set of doors-with-a-door, which caters to hobbit-sized people, kids - even pets.
Jjoo worked with Holitsch, a door manufacturer who helped them create the smaller doors within normal-sized doors. They remind me of a vertical version of the saloon bar doors in western films, only very kiddy-friendly. They've been surface-painted with marker-proof paint, so everything can be wiped off easily. Pretty clever stuff, though once your kids have grown up/pets moved to that grassy knol on the other side of town you may need to remove the door in favour of a normal, adult-sized one. [Jjoo via DesignBoom]