Gimp is an open-source, professional photo and image editor similar to Adobe Photoshop. It is 100% free and can be downloaded https://www.gimp.org/downloads/
Darktable is free, open-source, professional image editing software comparable to Adobe Lightroom. Darktable is available on Window, Linux and Mac, and can be downloaded here: https://www.darktable.org/
RawTherapee is a powerful, cross-platform raw photo processing system, comparable to Adobe Lightroom and Darktable. 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 that will benefit users who take the time to learn what it can do. Luckily the community is quite welcoming and helpful! Check out the Forum, read up on RawPedia, and ask questions – there’s always something neat to learn! :)
Official website: RawTherapee – Home