Darktable and the Gimp here as wellI use Gimp and Darkroom which are open source software. You can download and use them for free. They are not mainstream applications and some view them as having a steep learning curve. I am a Linux user so this was what was available for me and I learned to use it. It was probably easier for me because I had no previous experience with the mainstream applications. There are many tutorials on Gimp and Darkroom which is very helpful. @ronlane mentioned the two most popular programs on the planet and tons more guidance on them.