Rubber folder. Difficult to believe that it can be done, but this is made from recycled car tyres (or tires if you are from the US).
The folders come in any size. They are made and held shut by a pair of poppers, a simple but effective design that gives a natural capacity to the wallet. You can also see a binder whose cover is made from tyres in our recycled section, in the Inspiration side of our site.