Recommended tools to use on a daily basis
The Gimp
This software is the standard open source equivalent to Photoshop. It has the ability to define layers of settings on top of each others and also to apply layer masks and paint with brushes. Once a setting is applied on a layer, it becomes locked onto the layer unless you undo the history of actions, which makes it advisable to organize layers in a comprehensible and effective way.
This type of software is not only used for photo editing but also painting and image alteration in general such as matte painting, or even to make logos and web designs.
Darktable
Unlike Photoshop, Darktable is the open source standard equivalent of Lightroom.
It is specialized in post-processing photographs. Its advantage compared to Photoshop / Gimp is that it has no layers, the workflow consists of stacking settings without order, all settings are combined for the final rendering and you can alter / undo any settings at any time without caring for the order they were applied.
Also Darktable handles raw files out of the box, while Gimp alone cannot open Raw files.
Some advanced tools, especially functioning with histograms, such as color zones and contrast equalizer are only available in Darktable, but they may become available in Gimp in the future as it was the case for shadows and highlights since 2018.
What is open source
Open source is a way of licensing software that allows anyone to read and contribute to its code. Since I was a student pursuing a bachelor in computer science, open source struck me as an amazing way of sharing knowledge and building communities. It has some advantages such as increasing interest among potential developers to work on the software, companies can sell their services around an open source software and they usually give back to the community. They are following the philosophy of “sharing is caring” and in the end everybody can benefit from it.
Before quitting my job to work on this photo editing project, I was a backend website developer specialized in a content management system called Drupal. Not only the fact that it is open source helps it improve and grow over time thanks to the feedback and discussions among its community, but also its security is improved because anyone using it (and there are millions of people using it) could spot in its code source security issues and help to fix them. Malwares cannot live long in open source software because active open source projects are tightly monitored by their community and they are shared through trusted repositories.
Difference between free and open source
A free software is not necessarily open source but the opposite is true. Which means the security and community assets explained above do not often apply to free software.
Licenses
The GPLv3 license is used by Gimp, Darktable and many more open source software.
It was notoriously brought by Richard Stallman, who has been actively promoting the idea of open source since 1985. After many iterations, a license named GPL (Gnu General Public License), is now in version 3 and has been in heavy use since 2007.
In short, the idea of this license is that anybody can modify, redistribute and sell the software and code as long as the GPLv3 license terms remain bound to it untouched.
All updates included
One of your best bets with an open source software is that you will benefit from all its updates for free, by design. Also community support is usually very high.
While with Adobe software you would have to pay to upgrade them, users of Gimp and Darktable have been able to use them freely with more than a decade of updates !
Compatibility
Last but not least, open source is often developed on the Linux operating system (since it is itself an open source software, as opposed to windows and mac os), in turn you will often find out that open software is compatible with more devices and computers because of the way it is community developed, they release them on more systems.
The same cannot be said of Adobe software for instance, which are not natively supported by Linux operating systems (only by emulating windows).
List of other open source softwares and when to use them
Krita
Krita is comparable to Gimp and Photoshop, its appeal is for painting and using it with interactive pen displays and virtual brushes. I don’t advise using it in place of Gimp because the latter is better suited for photo editing than painting, especially also Gimp is much more performant when displaying preview of settings to be applied.
RawTherapee
RawTherapee is specialized in post processing raw file while Darktable handles jpeg and png files extension. Not only that but as I am writing this ebook, the community of developers around Darktable is much more active with a new version released in 2022, instead of 2020 for RawTherapee, which makes the former more appealing in the long run.
DigiKam
Digikam is not a photo editing program but can be used to manage, sort and filter your own photos by attributes such as EXIF data. It can be handy to analyze your photography styles and preferences evolving (or not) over the years and to compare the rendering difference across the different hardware (lens, phone, camera) that you use.
However, thanks to its collections feature, Darktable gives you the ability to do the same !