Recently, a lot of media attention has been given to various ways to increase storage space in a house. Wendy A. Jordan's book Making Room posits many possible examples, though most of them seem to come down to "knock out some drywall between beams and build shelving there" or "make the area under your stairs into a cabinet/closet/office/etc." This definitely won't work at my house, where the area under the stairs is the upper half of the stairwell that leads to the basement! No, we have to line the walls with shelving and hope for the best.
One of the coolest "increased storage" areas I've seen is in the family home of one of my friends, where there are two secret rooms. One isn't so much secret as "easy to miss" (it's a tiny room accessed from inside the garage), but the other is behind a bookcase. To get to it, you have to unload and move the entire bookcase, and he's always claimed it's "not that cool," but still: secret rooooooooom!
Now you, too, can have a secret room, by following this tutorial at WikiHow for a Hidden Door Bookshelf. You'll have a wall of useful shelving, and one of the units will hide a door! You're probably only going to keep all your valuables in your new hidden room, but it's so completely awesome that it's worth building anyway. (And that's a good thing: because you have to build a steel frame, it's a relatively complex project, definitely not for novices.)
Bonus: unlike my friend's hidden-room-bookshelf, it's built on a pivot, so you won't have to take everything off of it to get into the room itself.