GIMP's user interface used to be quite rough, and in many aspects that is still a case. So in 2006, we started collaborating with OpenUsability project and +peter sikking
of Man+Machine Works, a Berlin-based company that provides user interaction design services.
Over the years, a number of problems that GIMP's UI had were eliminated thanks to working with Peter: the free selection tool got a polygon selection mode, rectangular selection and cropping tool were improved by order of magnitude, a solid foundation was laid for upcoming Unified Transform and Warp Transform (on-canvas version of the iWarp plugin) tools, and the list goes on.
Even more, the idea to have active grabbing zones in the Crop tool found its way into more free/libre applications like +Hugin
(panorama stitcher) and +darktable
(photography workflow app and raw converter).
Unfortunately, all things come to an end, and Peter departed from the project. We thank him for working with us, and we are open to new collaborations to further improve user experience for the GIMP community. A point to start is http://gui.gimp.org/index.php/GIMP_UI_Redesign