Le Crimp (mostly en français) is a French collective that explores organic and abstract geometric [ I | II | III ] (PDFs) approaches to the art of origami. Read the white papers, browse the gallery or watch videos of artworks being made or being used in still-motion animations
MetaFilter's Eric Gjerde has just come out with a book, Origami Tessellations: Awe-Inspiring Geometric Designs. Eric also makes really unusual, complex and mathematically interesting origami. His site has all kinds of cool things to look at and explore, like Owen Jones' The Grammar of Ornament, Joel Cooper's amazing paper sculptures and Ernst Haeckel's awesome book, Kunst-Formen der Natur. [more inside]
What if Euclid had been Japanese? There are traditionally stated and proved theorems about origami. And MetaFilter has previously explored modular origami (as well as the boring old artistic kind), which has a geometric foundation. However, origami itself is a powerful mathematical framework that allows one to, for instance, solve the famously insoluable problem of trisecting an angle. More generally: Traditional geometry solves quadratic equations, origami solves cubic ones. (Many more mathematical items about and using origami can be found in the excellent mathematics teachers' book: Project Origami: Activities for Exploring Mathematics, most of which are unfortunately not findable online).
Astonishing geometric art using only folded paper plates, from Bradford Hansen-Smith at wholemovement. View the gallery of fantastic polyhedral creations, and learn how to do it yourself. (For more fun with paper plates, see also Paper Plate Education: Serving the Universe on a Paper Plate.)