GIMP (GNU Image Manipulation Program) is a free, powerful alternative to Photoshop. It is easy to use with a range of features for casual to professional users. It is an especially great choice for those with a low budget, or those looking to get into photo editing.
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GIMP is the most feature rich free image editing tool. It has most image manipulation and workflow tools that you would expect from an image editing software and what it lacks it makes up with plugins.
Pro Free, open source and cross-platform
GIMP is completely free and open source, meaning you can use GIMP and all of its features without spending a penny. This makes it an excellent case for artists or designers who may not have the budget to spend over 700 USD on Photoshop. It is also available for free on Windows, Mac OS and Linux. It's licensed under GPL with source code available here.
Pro Content-aware tools with a plugin
There's a content-aware fill plugin available for GIMP.
Pro Modular & single-window interface options
By default GIMP splits out each window, but you can check to use the one screen mode in options if you prefer.
Compared to to other photo editing software, GIMP is very light weight. In addition it isn't very resource-intensive, meaning you can put it on a flash drive and have it with you to load up on just about any computer.
Gimp has less features than programs like Photoshop, but for a beginner, or someone not needing complicated options, Gimp is the perfect choice. It allows you to quickly do basic photo manipulation so you can easily get the product you need without having to worry about complicated features getting in the way.
Pro Full channel support
Con Unintuitive interface
GIMP doesn't embrace OS X application design, thus Mac users might have a hard time wrapping their heads around GIMP's interface.
Con Less features than other programs
Being a free program, it shouldn't be surprising that it has limited features. For those needing more advanced features for something more professional, GIMP just isn't for you.
Con Lacks adjustment layers
Adjustment layers offer a non-destructive way of combining different photo manipulations. Without adjustment layers the only way to see changes is by irreversibly editing the image.
They are promised in future updates.