GIMP - GIMP is a multiplatform photo manipulation tool.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Pixlr - Pixlr® makes it easy to transform everyday images into stunning works of art.
Sketch - Professional digital design for Mac.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Affinity Designer - Professional creative software, exclusively for Mac.