Raytracing
http://fabiensanglard.net/rayTracing_back_of_business_card/index.php
The goal was to produce the source code for a raytracer...that would fit on the back of a business card.
Ray-Traced Sphere
http://www.traipse.com/sphere/index.html
"...trying to think of something disturbing to do with my circa 1986 Casio fx-4000P calculator. ... I programmed the calculator to assist me in ray-tracing a sphere by hand."