Several major changes have been implemented in the latest upgrade to the open-source freeware called The GNU Image Manipulation Program. Known as The GIMP, these changes include some midlevel user interface adjustments and improvements to several tools. Version 2.6.0 is also the first release that attempts to integrate GEGL, a graph-based image processing framework that allows for non-destructive image editing.
The GEGL integration is mostly a back-end change with a tentative implementation. In other words, the bugs are not necessarily all worked out. As such, it is not turned on as a default. You can use it in two places. Its color operations can be activated in the Colors menu by clicking Use GEGL. This will enact color changes in 32-bit floating point linear light RGBA, as opposed to the standard 8-bit.
The second option for exploring GEGL in GIMP 2.6 is the GEGL Operation tool. The technical explanation is that this applies GEGL operations to an image, with on-canvas previews of your edits. When you select the tool, it will give you a list of about two dozen global edits you can make to an image, including Gaussian blur, adding noise, and sharpening, all supported by the nondestructive GEGL code.
Although it worked fine when I tried adding noise, it crashed when I tried using the c2g grayscale tool.
Two tools have been improved. The Free Select tool now supports polygonal segments, as well as mixing those segments with freehand selections, and editing a selection area. The GIMP's changelog states that the free select tools is one of the most versatile in their toolbox, and I'm inclined to agree. Users can now map different brush parameters, such as size or opacity to pressure or velocity, using Brush Dynamics. This should result in better responsiveness to tablet input.
Interface changes include integrating the Toolbox menu bar with the Image Window menu bar, cutting down on clutter. You now can pan beyond an image border, too. There's also an option for quickly reopening recently closed docks--not a major change, but a useful one nonetheless.
The GIMP 2.6 has not yet been ported to the Mac. Full release notes can be read here.