Ever since I saw a tutorial for creating shelving out of cardboard on Instructables, I've been using the principles to create shelving and storage all over my house. Layered and/or interlocking pieces of cardboard - with the corrugations running vertically - are incredibly strong, yet light-weight, free, and easy to work with. All you need are some scissors, glue, tape, an exacto knife, and cardboard.
In the kitchen, above my stove to hold my spices, salt & pepper shakers, and toothpicks:
In the kitchen, two corner shelves with Coca-Cola bags as the tops/bottoms of the shelves:
Solving the wasted space/where to put the DVD player; covered in old comic book pages:
More comic book pages cover this one, which is screwed to the wall to support the weight of the movies:
A special yin-yang one for the baby's room, inspired by one on Instructables:
These shelves are so fun to make that I plan on making more - I'll post photos as I do.