Reutersvärd Triangle
This triangle was created by Oscar Reutersvärd in 1934, the first version of the Penrose triangle. There is a common 3d printing design that implements this triangle. I adapted this design into modular origami. If you can't figure out how it's implemented, check out the links for a side view.
Challenge
Optical Illusions
Diagrams
Reutersvärd Triangle diagrams