GIMP is the open source alternative to Adobe Photoshop that can be used for anything from simple digital photo editing to complex original art. It is a robust application by itself, but it can be enhanced exponentially with the addition of third party brushes, scripts and filters.
Brushes allow you to add interest to your artwork. They either work with a single click, or by moving the mouse across the work area. GIMP comes with a fairly large set of preinstalled brushes and provides the ability to create your own. Before you start creating new brushes, you should check out the thousands of brushes already created and freely available. Not only can you use the brushes created for GIMP, but GIMP 2.6 also allows you to use Photoshop brushes natively. You can find many brushes on sites like DeviantArt, Techzilo, and Blendfu.
The brushes are usually downloaded as sets in the form of a zip file. Once the file is unzipped, you can move the .gbr (GIMP) or .abr (Photoshop) files into your brush folder. The brush folder can be found at ~/.gimp-2.6/brushes. Start or restart GIMP and the brushes will be in the brushes dialog ready for use.
Scripts and Plugins
You can also extend GIMP by installing optional plugins and scripts. Dozens of scripts and plugins are installed with the default installation, but many more can be found at the GIMP Plugin Registry and at Deviant Art.
GIMP has its own scripting language, called Scheme, that is used to create Script-Fu scripts. You can identify a script-fu script by the .scm file extension. These scripts can be moved to the scripts directory found at ~/.gimp-2.6/scripts.
Plugins are written in C, Python or Perl. The plugins must be installed to be used. Often installing the plugin simply means uncompressing it and moving it to the ~/.gimp-2.6/plugins directory. However, there are some plugins that have to be compiled before use. Always read the accompanying documentation for the correct installation procedure.
Not only can you use brushes designed for Adobe Photoshop, you can also use Photoshop filters as long as you install the PSPI Plugin and Wine installed. Install the pspi plugin and the pspi.exe.so files in your ~/.gimp-2.6/plugins directory. You can download Photoshop filters from Flaming Pear or Photoshop Filters. The filters should be placed in a filters directory outside of the gimp-2.6 directory. When you open GIMP, point the PSPI script to the correct location by clicking on “Filters → Photoshop Plug-in Settings" and providing the path to the filters directory.