GIMP (short for GNU Image Manipulation Program) has to be one of the best graphics editors for Linux that I have came across. A lot of people that I know choose not to use GIMP and prefer Photoshop, due to the wide range of downloadable brushes, and extra add-ons etc.
What they do not realise is that there are thousands of downloadable brushes that are also available for GIMP too. Below are a collection of some of the best ones that I find myself using.
To download the following brush sets, simply click on the image to download the ZIP file. To install them, just copy the contents of the ZIP file into your GIMP Brushes directory (~/.gimp-2.6/brushes/)