GIMP is a free program for such photo retouching, image composition and image authoring. Essentially it's the closest thing to Photoshop you can get for free.
Krita is a professional Free and open source (FOSS) painting program. It is made by artists that want to see affordable art tools for everyone within concept art, texture and matte painters, illustrations and comics.
Paint.net is a free and very capable image editing software for Windows. Great alternative to Photoshop for people that do not need all the stuff PS offers.
Affinity Photo is a professional, full-featured raster graphics editor. Working in Affinity Photo is always live and you can pan and zoom at 60fps, with live previews and non destructive application. It supports unlimited layers and a dedicated RAW editing workspace, as well as...
Online photo editor, which can work with PSD, XCF and Sketch files (Photoshop, Gimp and Sketch App). Convert between PSD, XCF, JPG, PNG and many other formats.
Pixlr® makes it easy to transform everyday images into stunning works of art. Whether you’re applying a quick fix to your photos or adding your personal touch with effects, overlays, or borders, Pixlr has everything you need to make your moments beautiful.
Glimpse is an open source image editor based on the GNU Image Manipulation Program. The goal is to experiment with new ideas and expand the use of free software.Discontinued
The Glimpse editor has been discontinuedSource: https://glimpse-editor.org/posts/a-project-on-hiatus/
PhotoFiltre 7 is a complete image retouching program. It allows you to do simple or advanced adjustments to an image and apply a vast range of filters on it. It is simple and intuitive to use, and has an easy learning curve.
PicMonkey is a feature-rich online photo editor that works right in your browser; no downloads necessary. Touch-up tools like Teeth Whiten, Blemish Fix, Weight Loss. Effects like Orton, Cross Process, Color Boost, with drag and drop ease.
Meet the most innovative professional photo editor built by photographers for photographers. Luminar brings everything you need to make perfect photos in less time!