GIMP version 2.4 or higher supports the Photoshop .ABR brush format! Files must be copied to the directory /gimp-2.x/brushes/ (where x=version number) or /usr/share/gimp/2.0/brushes/.
Stunning Gimp Brush - spruce twig is striking in its beauty. There's everything and needles and cones, and branch. So feel free download and create their masterpieces.