If your house has more than one floor, chances are good that there is a lot of wasted space above and below your stairs that is just crying out to be filled with stuff. Shelves underneath the stairs are easy, and something that I added when we first moved in. But after years of climbing up and down the steps and staring at all of that empty white space up there, I finally had to do something with it, so as a side project to my current family room remodel, I've conjured up an entirely new room right off the kitchen that I'm calling a butler's pantry.
The floor is a piece of 3/4" plywood made just a bit beefier with some 1x4s, connected with some heavy duty hinges to a 2x4 attached to the wall with lag screws into the studs. It rests on a matching 2x4 on the opposite wall, and I cut a notch out of the wood to allow room for the handrail. This was designed to hold a single person, but we've already tested it with two adults (300+ pounds) and it had no trouble holding us.
Once the floor is in place, you can assemble whatever kind of shelves or hanging storage containers you like. I used 3/4" pine boards that will easily hold 50-100 pounds on each shelf. Add a few hooks to the underside of the shelves and you have a place to hang pots and pans, or dry herbs from your herb garden. Mmmm... smell the rosemary. For just a couple hundred bucks in lumber and hardware, maybe you should add a new pantry to your house too? Eat your heart out Pinterest!
[Update: 3/11/18] I added a couple of recent shots to show the current state of the pantry.
[Update: 12/28/22] Added a close-up of the hinges.