Here is a list of useful open-source software dedicated to image processing, graphics creation and photo retouching, which you can download and use for free. Most of them are available for multiple architectures and platforms.

Raster image editors

logo_gimp GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.
logo_krita Krita is a free digital painting and illustration application. It offers CMYK support, HDR painting, perspective grids, dockers, filters, painting assistants, and many other features you would expect.
logo_mypaint MyPaint is a fast and easy open-source graphics application for digital painters. It lets you focus on the art instead of the program. You work on your canvas with minimum distractions, bringing up the interface only when you need it.

Vector image editors

logo_inkscape Inkscape is a free and open source software vector graphics editor. Its goal is to implement full support for the Scalable Vector Graphics (SVG) 1.1 standard.
logo_scribus Scribus is an open source program that brings professional page layout to Linux, Windows and other desktops with a combination of press-ready output and new approaches to page design.

RAW image developers

logo_darktable Darktable is an open source photography workflow application and RAW developer. A virtual lighttable and darkroom for photographers. It manages your digital negatives in a database, lets you view them through a zoomable lighttable and enables you to develop raw images and enhance them.
logo_rawtherapee RawTherapee is a cross-platform raw image processing program, written in C++, using a GTK+ front-end and a patched version of dcraw for reading raw files. It is notable for the advanced control it gives the user over the demosaicing and developing process.
logo_photivo Photivo is a free and open source (GPL3) photo processor. It handles your RAW files as well as your bitmap files (TIFF, JPEG, BMP, PNG and many more) in a non-destructive 16 bit processing pipe with gimp workflow integration and batch mode.
Lightzone-logo LightZone is a free, open source digital photo editor software application. It was originally developed as commercial software by the now-defunct Light Crafts. Its main purpose is to handle the workflow when handling images in various RAW formats.
logo_rawstudio Rawstudio is an open-source program to read and manipulate RAW images from digital cameras.
PhotoFlow is a fully non-destructive photo retouching program providing a complete layer-based workflow including RAW image development.

Command line tools

logo_gmic G’MIC is an open and full-featured framework for image processing, providing several different user interfaces (including a command-line interface) to convert/manipulate/filter/visualize generic image datasets, from 1d scalar signals to 3d+t sequences of multi-spectral volumetric images.
logo_im ImageMagick is a software suite to create, edit, compose, or convert bitmap images. It can read and write images in a variety of formats (over 100).
logo_gm GraphicsMagick is the swiss army knife of image processing. it provides a robust and efficient collection of tools and libraries which support reading, writing, and manipulating an image in over 88 major formats.

Online image manipulation

logo_gmicol G’MICol (stands for G’MIC Online) is a web service offering free effects and filters you can apply online on your photos and pictures (color effects, filtering, framing, geometric distortions and so on…). It is based on the G’MIC image processing framework.

3D computer graphics

logo_blender Blender is a professional free and open-source 3D computer graphics software product used for creating animated films, visual effects, art, 3D printed models, interactive 3D applications and video games.