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.
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.
Compared to to other photo editing software, GIMP is very light weight. In addition it isn't very resource-intensive, meaning it is possible to put it on a flash drive and have it with you to load up on just about any computer.
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.
Krita's preset brushes are one of the default dockers. Each brush has a preview on mouseover that shows a detailed view of the type of brush involved. All the brushes also have useful and descriptive names such as "HP Pencil" or "Textured Fuzzy".
Since releases are so often and with little time between them, usually new features are not tested a lot and this can bring a lot of bugs with them. Which fortunately are quickly patched in the next release.
Krita's official documentation is incomplete in some areas, especially for new features that are constantly added. But this is compensated with it's great design and usability which makes it easier to understand how things work.
Pinta is a simple and lightweight alternative to popular image editing tools. It's very similiar to WIdnows' paint.NET. While it does have the most basic features and functionalities, it's missing some of the more advance features that can be found elsewhere.