In the master bedroom closet I've got a cedar chest filled with stuff. We store our shoes under the dressers, and I have the bed lifted up on risers for more storage space. In the kids closets I use plastic storage bins for outgrown clothes waiting for the next user, off season stuff, toying in rotation and general storage. There's lots of room under the baby's crib for small storage boxes too.
I put shelves above the washer/dryer for cleaning supply storage, and we keep tools under the bathroom sink cabinet. Over the door storage units are great for cookbooks, hats and mittens, shoes or even as a food pantry. There's enough room under the futon to store 2 folding chairs for extra seating when we have guests, and a small bookcase doubles as an endtable. We have fairly spacious bathrooms, so I use plastic drawer storage units there for odds and end things - I store my knitting and yarn supplies in the guest bathroom! If you look hard you can find shower curtains with pockets or you could make one yourself and have extra space for bathroom sundries.
Don't forget about the dead space between the wall and your appliances. Hang cup hooks or the like and easily store your brooms, mops and yardsticks. Get a hanging battery holder to keep track of the batteries and keep them out of your junk drawer.