GIMP

Highly Recommended

GIMP (GNU Image Manipulation Program) is an open source image editor. Whether you need simple color correction to a photo, automatically seamlessly tiling textures, or custom a powerful image generator for various noise or fractal patterns, Gimp has a myriad of tools. It is available for Windows, Linux, and Mac

GIMP Website

Tutorials

Competition

Adobe Photoshop
Krita

Features

• Layer-based image editing
• Large selection of noise and pattern generators
• Select from a variety of brushes, or build your own
• Create vector-style paths
• Large number of addons, scripting framework for languages such as C, C++, Python, and more.

What It's For

GIMP is for manipulating images, and photographs.It has a dizzying array of noise and pattern generators. Compared to Krita, GIMP is more geared towards the dual purpose of photo manipulation, and generating custom mathematical models with its scripting framework. While GIMP does have an adequate brush engine, Krita is usually the better tool for drawing images from scratch.

Usability

GIMP is intuitive and responsive. If you are coming from the world of less-powerful image editors, such as the freely bundled Microsoft Paint, GIMP is easy to understand. The tools all do what they are supposed to. The interface was originally inspired by Photoshop, but features a wide degree of customization. You can either keep your entire workspace in one window, or undock tool panels and move them about freely.

Sometimes, GIMP suffers from certain tasks freezing the interface for long periods of time, such as when opening the Export dialog. These issues seem to come and go with different updates.