You already know GIMP, as it comes pre-installed with half Linux distributions or you may use it in Windows. It has a ton of features but it doesn't come with many brushes to boost your creativity even further. No problem! You can simply add brushes made by professional artists very easily and for free. Some great sites are Qbrushes.com, Brusheseezy, PSbrushes, deviantART GIMP brushes, deviantART PS brushes..
-What?? It says "Quality brushes for Photoshop"!
That is not an issue as GIMP can use Photoshop bruses. Simply download any brush you like, uncompress it and copy it to /home/username/.gimp-2.6 (this will vary according to username and version). Next time you launch GIMP, you can select your new brushes and make great designs.
While I was adding brushes, GIMP refused to start. A brush set was the issue, so I deleted it and GIMP continued to work fine. The other 10 brush sets I downloaded, worked with no issues.