GIMP 2.8 is a solid photo editing and graphics program as many of you already know. The work is ongoing toward the 2.10 however, and the development release is somewhere around 2.9.4 and is already quite stable.
2.9 has support for 16bit and higher, and has the #MyPaint brushes engine integrated.
GIMP 2.8 (stable) is still actively maintained and is available for Linux/Win/OS X.
As for 2.9, nightly builds are available for windows. There are the options of an unofficial PPA out there for linux that you can use at your own risk, if you don't want to build it yourself, and flatpak packages are apparently in the works.
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