Generative Pattern Maker
Here's a web-based generative pattern maker that produces images like these:
You can use the online version of the tool to make your own patterns. It also works offline, so you can just download the code and open the HTML file in a web browser.
Click the 'START' button to begin a new pattern. You can pause at any point, or just let the pattern run to completion. Running the 'PENCIL' tool produces a nice shading effect (shown in the images above). If you produce something beautiful you can save a copy using the 'DOWNLOAD' button.
If you select the 'Continuous' option then each time a pattern is completed a new one is started automatically.
Each pattern is based on a 'seed' number that can be used to reproduce the same pattern again. The three most recent seeds are shown at the bottom of the page. Clicking a seed number will generate the corresponding pattern again.