The Foldscope is an origami-based optical microscope that can be assembled from a flat sheet of paper in under 10 minutes.  Although it costs less than a dollar in parts, it can provide over 2000x magnification.

Cybulski JS, Clements J, Prakash M (2014) Foldscope: Origami-Based Paper Microscope. PLOS ONE 9(6): e98781.