Download dirty-brushes.zip in this google drive folder.
Set of 49 brushes. Most of them have some kind of randomness going on. They are fairly natural and try to mimic traditional tools. There are some loose groups in the set where all brushes have a similar style like wet, rough charcoal, Pointilism (correct word is "stipples")...
The google drive folder also contains a text file with more information, preview images and the .ora of the image above. Thanks to gdrives inbuilt version control the files are always the latest and the link should work even when I update them. Now it even includes a license readme.txt going like
Copyright 2016 Anna Timm
Anna Timm has waived all copyright and related or neighboring rights to all content in this Brush pack
See http://creativecommons.org/publicdomain/zero/1.0/ for details.