This is a great little shelf to put where space is at a premium. You can easily put the shelf up or down as needed. And you can make this one in minutes!
All you need are a couple of strap hinges, 2 pointed rods and a board for the shelf. And, oh yes, you'll need screws to attach everything.
Screw the hinges to the shelf and the wall - you may want to use mollies for stability. Make sure to not put a screw in the end holes of all of the hinges.
Use some pointed rods to hold the shelf up. When you don't need the shelf, remove the rods to let the shelf fold down on the wall.