Even the tiniest spare closet can become an office with the addition of a desk and some lighting. (Note the cord trailing out above -- the closet doesn't need to be wired.) Simply close the door when you're done working and keep those paper piles out of sight.
A coffee table can easily provide hidden storage; just lift the top and store blankets, magazines or board games. Corral remotes and snack dishes in a tray on top so they're easy to remove in a jiffy.
Tuck baskets into any "dead space," like under a bench or table. Since the furniture itself is already taking up square footage, you might as well squeeze some extra function out of that space.
A bed with built-in drawers is perfect for kids' toys or in an office that has to double as a guest room. In an office, you can stash extra file folders and envelopes in one drawer and guest linens in the other.
My rental kitchens have either been tiny and cramped or medium-size but with lots of unused space (tiny countertops with an awkward empty corner, for example). If yours is the latter, take cues from a grander home and bring in an armoire. It offers tons more storage than a kitchen cart does, plus it has doors to hide the mess. Use the shelves as a makeshift pantry or as a desk space, like in this kitchen.
A nightstand with drawers can hide just about anything. Besides the usual bedside items, you can stash spare linens, photo albums or even socks and other small items.
Adding a credenza in the living room creates a surface for lighting and a place for hidden storage. In this space it works double duty for the living room and dining area, but you could place one against the wall to store books, games or even off-season decoration.
A beautiful dresser or chest in the entry (even if your entry is just a hallway) can be a great place to stash gloves, hats, leashes and spare change. It could even be a good place to charge your phone and camera.
Storage under a children's play table provides a perfect hiding spot for craft supplies.