My cats have loved shoeboxes their entire life. Even when they were older and fatter, my cats always squeezed into the nearest shoebox. I loved seeing them with their fat hanging over the edge of the box.
Sometimes I'd pick up the box with them in it and carry them around.

Pros: cheap, find them everywere

Cons: No problems

Line it with a crinkle type blanket and it makes a cheap cat box for napping.

Pros: Any size will do

Cons: None

My kitten enjoy standing, playing, and sleeping in shoe boxes. I am thankful that he does not chew the rim of the box, like my other cat. A shoe box is a simple toy that provides a lot of joy to my kitten.

Pros: Inexpensive, cat enjoys sleeping in it.

Cons: May not be suitable for very large cats.

Shoe boxes are a great place for cats to hide in. My cat likes to wad up in the shoe box and take a nap. I like the fact that the shoe box can be recycled if it gets chewed up or ripped apart.

Pros: have them at the house, can recycle

Cons: can be chewed up

I'm so amused by our cat when he tries to fit his big fat body into a too small cat box. There must be something about the smell of cardboard because he loves to chew the boxes too.

Pros: fun, cheap

Cons: none