You can run book shelves along the upper parts of the walls around a room. It looks interesting and uses space that you otherwise wouldn't make use of.
Those over-the-door shoe holders can be great for storing a variety of things. We use one for craft supplies, and I saw a clever idea where someone did the same thing but put all the supplies in old peanut butter jars. It kept things very neat and organized. (Loose stuff in the pockets tends to attract other, non-related, loose items.)
I just saw an awesome idea for using a closet as an office. Basically, pop out the bar, put in a small table or built-to-size shelf, place shelves above for office-related storage, and pull up a dining room chair when you want to sit down to work.
Use shelving that runs floor to ceiling rather than lower pieces. You'll use less floor space this way, and it will pull the eye up and make the space look bigger.
Choose items that have duel purpose. Like a fold-out couch that can be used as a guest bed, or a bed with drawers underneath that can provide storage.