GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.
Download Link: https://www.gimp.org/downloads/
Learn how to use GIMP here on YouTube:
darktable is an open source photography workflow application and raw developer. A virtual light-table and darkroom for photographers. It manages your digital negatives in a database, lets you view them through a zoomable light-table and enables you to develop raw images and enhance them.
Download darktable for Windows: https://www.darktable.org/install/#windows
Download darktable for Mac: https://www.darktable.org/install/#macos
Learn how to use darktable here on YouTube: https://www.youtube.com/user/audio2u/videos
Open and edit your RAW digital image files with this free program: RawTherapee
RawTherapee is a powerful, cross-platform raw photo processing application, released as Free Software. It is designed for developing raw files from a broad range of digital cameras and targeted at users ranging from enthusiast newcomers who wish to broaden their understanding of how digital imaging works to professional photographers.
RawTherapee provides a powerful suite of tools for you to produce amazing photos and showcase your creativity.
RawTherapee benefits users who take the time to learn what it can do. Luckily the community is quite welcoming and helpful!
Download RawTherapee here: https://rawtherapee.com
Learn how to use RawTherapee here on YouTube: