Storage is always at a premium, especially in a small space. And while you can't add closets, what you can add are shelves. If a little extra storage is just the finishing touch your small space needs, consider adding shelves in one of these unexpected places.
Above the Door
If you've got a door in a corner (like the one in Whitney and Adam's Venice Beach home), you're looking at potentially several square feet of extra storage space. The great thing about a corner shelf like this one is that it can fit things that are too deep for traditional wall-hung shelves.
Above the Door (Reprise)
Even if it's not in a corner, any door can be a great spot to add a shelf provided there's a little space above the top of the frame. Spotted on Shelterness.
Above the Couch
The space above your couch is a great spot for hanging art, or, potentially, a great spot for a little extra storage (as seen in this living room from Whistles). Just make sure that your shelves are high enough above the couch that you won't whack your head.
Above the Radiator
Radiators are lovely because they make your house nice and warm, but also annoying because they limit where you can put furniture. Recapture some of that space by hanging shelves above your radiator, as seen on VT Wonen.
Along the Top of a Hallway
A single shelf hung along the length of a hallway (like this one from Hus & Hem) can provide a great deal of extra storage. As long as it's above head height, a shelf like this can work in even the narrowest of hallways.
Above the Television
Shelves above the TV (like these from Femina) have the additional happy effect of reducing the visual impact of the television itself, because everyone knows that bookcases (or shelves + pretty books) > electronics.
Above a Desk
Shelves above a desk are a great place to store reference materials, as seen on Style Me Pretty. (Although if you reference them often you might want to turn the spines the right way.)
Above the Bed
Not recommended if your area is prone to earthquakes, but otherwise eminently practical. This lovely example is from Brit + Co.
In that Little Spot Above the Cabinets
Storage above the cabinets is nothing new, but this example from Megan Pflug (lead image above) takes things to a new level by adding another shelf (propped up on modular wine holders) above the cabinets for double the storage.