Matthew Epler's Designing Generative Systems video series shows the process of programming a design system that creates visual elements based on controlled randomness. This project (definitely a work-in-progress) expands on the original concept by ceding some design control over to the user/viewer. They can select colors, layout and complexity, then export the pattern as an SVG file for printing or further manipulation with Illustrator or other graphics software.
To generate and download your own patterns, go here. The code for this project can be found at my GitHub repo. Finally, if you're interested in creating algorithmic art for the web, p5.js is a super capable, relatively friendly JavaScript library and The Coding Train is a quirky and invaluable resource.