I live in a really small house--about 500 square feet of living space located over my garage/workshop. For reference, that is 1/10th the size of Paula Abdul's clothes closet. So, every little corner of storage and living space counts. I try to think in terms of volume, not area, when it comes to organizing. You don't store three-dimensional stuff in two-dimensional square footage.
Enter the window seat storage shelves. The bay window had an unused volume under the narrow window seat/shelf, and my bookshelves are always overloaded. After some crude exploration with a drill bit, and then some fancy exploration with fiber optic camera I borrowed, I started making a mess. Cutting drywall in your bedroom is always a mess.
I discovered: two layers of drywall, some funky wallpaper, and tongue-and-groove redwood paneling. All of which met a combination of sabersaw, sawzall, and chisel.
With the space opened up, I had six cavities between the window framing, and the end ones were pinched off by the tapering ends of the window "bump." I used those end cavities to hold in-wall speakers, with drywall forming the face and sides of the speaker "box." I built shelves in the other four cavities. The bottom row is sized for DVD's and medium-sized paperbacks, and the top row can hold pocket-sized paperbacks two-deep (volume again!), a trick my grandfather used when building shelves in his house in Waterville, Maine. Clever guy.
The sliding doors are 12mm maple plywood, and they slide on shower door guides. The pulls are Ives brass finger pulls, in a satin nickel finish. After obsessive re-measuring, I cut the pockets for them with a spade bit, and glued them in place with a dab of painter's caulk. With all the weird dimensions and out-of plumb that comes with a 110+ year old house, I had to make most of the trim and baseboard parts from scratch, but found a pretty piece of pre-carved trim for the top border.
With the speakers on the ends, it made sense to move my receiver/amplifier into one of the shelves, so I wired two outlets on the right side, including one with a USB transformer, so I can charge my iPhone while it's also plugged into the stereo. There's no good access to wiring in that corner of the house, so I used a 10 amp, panel-mount C-13 electrical plug with a fuse and filter (just like the one on the back of a desktop computer). It's a small detail, but it lets me close the doors without unplugging anything--a much cleaner look than running an extension cord through a partially-open door.
In total, I moved about twelve feet of books into the new shelves, in addition to tucking away my stereo and speakers. Total materials cost was around $400, with about half of that used for the speakers and the transformer outlet.