GIMP

The Free & Open Source Image Editor This is the official website of the GNU Image Manipulation Program (GIMP). GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and mo...
Language(s): C
Operating system(s): Linux, macOS, Microsoft Windows, BSD, Solaris
License: GNU GPL v3+

Inkscape

Inkscape is professional quality vector graphics software which runs on Windows, Mac OS X and GNU/Linux. It is used by design professionals and hobbyists worldwide, for creating a wide variety of grap...
Language(s): C++ with gtkmm, Python (extensions)
Operating system(s): FreeBSD, Linux, Mac OS, Windows
License: GNU General Public License, version 3 or later.

Krita

Krita is a FREE and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry. Krita has been in development for over 10 years and has had a...
Language(s): C++, Qt
Operating system(s): Unix, Linux, Windows, Mac OS
License: GPLv2 or later.

LightZone

LightZone is professional-level digital darkroom software for Windows, Mac OS X, and Linux, that includes RAW processing and editing. Rather than using layers in the way that other photo editors do, L...
Language(s): Java
Operating system(s): FreeBSD, Linux, Mac OS X, Windows
License: BSD license

RawTherapee

RawTherapee is a cross-platform raw image processing program. RawTherapee involves the concept of non-destructive editing, similar to that of some other raw conversion software. Adjustments made by th...
Language(s): C++ gtkmm
Operating system(s): Linux, FreeBSD, Windows, Mac OS X
License: GPLv3 (as of version 3.0 alpha 1), proprietary (up to version 2.4.1)