PixelPlant

Procedurally makes plants with pixels and L-Systems! Press F5 for a new plant. More features (including a UI) coming soon to tweak the generation settings! Right click to save image.

Source on GitHub. The tool itself can generate a *lot* more than what's shown, and the UI, once functional (I don't know how to do front-ends lol), will allow you to edit the rules, save and import rules, and more! Check out the Wikipedia page for L-systems for more information on how the plants are made, mathematically.