Origami picture frame, folded from a single sheet of paper without any cuts or glue. There is a slit going around the inner perimeter of the frame where you can insert the picture, and the tension of the paper can hold the picture in place. This particular frame, suitable for a standard 15×10 cm picture was folded from a strip of F-Color paper with dimensions roughly 70×10 cm. 70 cm is the shorter side of a B1 sheet, which is quite convenient.
This frame can be made with arbitrary proportions: you can vary the width, height and depth as well as the breadth of the border independently. It is also possible to make a modular version which is easier to assemble and can be made with smaller sheets of paper, but then it becomes modular origami rather than a single-sheet model.
I was inspired to design this model by a workshop held in Erkner (German Origami Convention) by Hans-Werner Guth who taught his Trapped Cube model. His work, in turn, was inspired by the bent frames of Thoki Yenn.
Oh, and you can also place an origami tessellation inside a frame like this in order to take nice back-lit pictures (you need a bit of margin on the tessellation in order to insert it into the slit, though).