The GNU Image Manipulation Program is a great Open Source and crossplatform image editing tool, and I say that after many years using only (pirated) Adobe Photoshop. However in the last years I decided to move all my works to GIMP and now I use it as my main tool as a freelance Graphic Designer. It was not easy at first due to its quirks but now with the upcoming launch of GIMP 3.0 things can't get any better!
So here I will list tools and general resources that can be used in conjuction with GIMP in you graphical productions, check it out!:
Custom modifications with better shortcuts, interface and so on.
Pre-made brushes using the .gbr format.
Since most of the brush packs out there are hosted on DeviantArt and require a Log In to be downloaded I have made a copy of the ones I use avaliable here.
Pre-made color palletes using the .gpl format.
Repeating tile images
I also have more patterns avaliable here.
Custom UI color themes for the program
Software that can be used together with GIMP to extend its functionality