To really maximize your space, look in places you normally may not think of.
Take advantage of space between wall studs to carve out a shallow display shelf. I did this in an upstairs hallway between two bedrooms. A narrow expanse of wall created an awkward space; centering a built-in shelf on the wall fixed the proportion problem and gave the wall a new purpose.
Try adding a shelf above a doorway (great for bathrooms – put some pretty baskets on the shelf to hold various toiletries while keeping them out of sight).
To make use of empty space under a window, try a low bookcase. Or build a window seat with a hinged lid for even more storage and seating options.
Many people have wasted space under their stairs that can be put to good use as well. This is a great place for built-in cabinets or shelving.