GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes.
Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins.
GIMP is the most "competitive" freeware alternative to PhotoShops. It is available in Mac and Windows and Linux. Actually the source code is available for anyone to modify and use.
It does pretty much most of the things Photoshop can do but it's not exactly Photoshop. For most casual users or small business GIMP is much more than enough. It has an extensive collection of tutorials for user to quickly learn how to use it. If you search on YouTube with "GIMP Tutorial" you will find a lot of very useful Tutorials to help you to learn to use the software.
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Official Site: https://www.gimp.org/
License: GNU/GPL license.
OS: Windows, Mac, Linux