Cardboard is awesome. It's the perfect material for your imagination. Before you recycle your latest collection of boxes hand them over to your kids or help them create something of their own!
Ikat bag has a post on several different things they've made from cardboard
Not quite a fort but a great place to play from 45wall Design
Love this 2 story clubhouse from bennelson on Instructables
A simple banana box fort from Cardboard Collective
Love Fort Rex from Kat This Kat That
An epic fort with multiple entrances from Supriseaholic
So they can play in the dryer (which my boys always wanted to do) from Estefi Machado
Do you save your cardboard for the kids to play with?