Hanno's original coffee ring page from 2009. https://web.archive.org/web/20100719202509/http://hanno-rein...

I'd say the best ones are created by coffee...

There was the GIMP Coffee stain filter (though it looks like it is not included in Gimp 3). https://docs.gimp.org/2.10/en/script-fu-coffee-stain.html