Well, browsing around digg.com I found a great link for all open-source lovers, especially GIMP users. 35 free artistic brush sets.
And what better way for me to admire GIMP than by presenting a list of some amazing, beautiful and stunning brushes? The name, link and description of each brush are given just below each illustrative image. There are also sections linking to huge GIMP collections and further resources aimed at every group from novice to expert."