Cellular automata are very simple, rule-based systems that can create remarkably complex and organic patterns. I adapted one to design celtic knots. Unfortunately, the designs are not really ammenable to online display because the images are huge and highly detailed.
My first real piece of this type was done for Andy Sloss who invented the knot design technique that I use and is the basis for all of my work. I had the pleasure of collaborating with him to a degree on this project because the celtic knot font that he also created made the software possible; however, there were some complications in the Mac->PC transition of the font that he resolved for me so we began corresponding directly. He has done some very interesting things with the design as well that are quite stunning. This is from the electronic image -- the final was printed out at a high resolution at about 8"x24"