If you have stairs in your house, don't disregard the space underneath them.  Here are some ideas for maximizing the space under your stairs.

ussophoto courtesy of ctd2005 flickr.com/photos/kikisdad/25543722/sizes/m/in/photostream/Closet
Most everyone has a closet under their stairs.  It is a bit of an awkward shape, but you might be able to store your vacuum cleaner there and perhaps hang a few coats.

Bookcase or Drawers
Near the start of the stairs, there is very little height to store things.  Consider putting a short bookcase or deep drawers in that area to maximize the storage space. 

Bathroom
If you have enough height, you can add a small powder bath under your stairs.  Tuck the toilet in the shortest part of the room.

Wet Bar
A wet bar does not take up too much space and would be perfect under the stairs. 

Entertainment Center
An entertainment center can be built in to the shorter portion underneath your stairs.  All of your components can be wired from behind if you also have a closet in that area.

Home Office
If you use your home office for paying bills and checking email instead of full time work, you may not need too much space for a home office.  You can tuck your home office under your stairs to keep it out of the way.  If your home office is enclosed under the stairs, make sure there is proper air conditioning in that room.  You may want to change out the solid door with one having glass so the room does not feel so enclosed.

Playroom
Kids love having their own space.  Use the area under your stairs for kids to play without getting in the way or having their toys all over the house.

Wine Cellar
You can have a wine cellar under your stairs.  Make sure to have proper air conditioning to keep the wine happy.

Hidden Room
Everyone loves a hidden room.  Put a bookcase on a heavy duty hinge under your stairs to create a hidden room.  You can store your safe, guns, and other valuables there.

Pet's Room
You can create a small room for your pet under your stairs.  Make sure to provide adequate air circulation and a door with a view.