Pepakura is a program that creates buildable papercraft models.
Pepakura Viewer is a free software, dedicated viewer that shows 3D models and patterns for papercraft created by other people in Pepakura Designer from Tamasoft.
Such a model can be unfolded to a flat template printable on paper, to be cut afterwards, folded and glued together. There are many features in the Viewer to help you show and print your creations and construct paper models.
Pepakura Designer allows you to scale and print larger 3d models for things such as helmets, armor, etc.
I'd show off some of the cosplay armor/projects I've made in the last year or two as well.