I 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.