In many homes there are doors that open into a small closet-spaced niche designed to only hold the opened door and provide access to the room. Other than hanging a poster, there isn't much you can do with that space, but if you have kids or just want to display magazines you can add narrow bookshelves to keep reading material handy.
Interior design weblog Design Sponge highlight an apartment by contributors Joshua and Jodie Steen that includes the custom-built narrow bookshelves designed by Joshua. Unfortunately no plans were included but seems to be an easy-to-replicate design using 1x2 boards -- just make sure you measure everything twice to ensure you'll have enough door clearance once you fit the shelves. If you look at the above photo you'll see how the door's knob fits into an opening in the shelves.
Sneak Peek: Joshua and Jodie Steen [Design Sponge]