GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software suitable for such tasks as photo retouching, image composition and image authoring. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, a image format converter, etc.
GraphicsMagick is a robust collection of tools and libraries which support reading, writing, and manipulating an image in over 90 major formats including popular formats like DPX, DICOM, BMP, GIF, JPEG, JPEG-2000, PDF, PNG, PNM, SVG, and TIFF. A high-quality 2D renderer is included, which provides a subset of SVG capabilities. C, C++, Perl, Tcl, and Ruby are supported. Originally based on ImageMagick, GraphicsMagick focuses on performance, minimizing bugs, and providing stable APIs and ABIs. It runs on all modern variants of Unix, Windows, and Mac OS X.
Raster3D is a set of powerful tools for generating high quality raster images of proteins or other molecules. The core program renders spheres, triangles, cylinders, and quadric surfaces with specular highlighting, Phong shading, and shadowing. It uses an efficient software Z-buffer algorithm that is independent of any graphics hardware.
Gimp User Filter is a work-alike to Adobe's Filter Factory. It is a generic image processing plug-in that can be used to create new filters for The GIMP without having to go through the process of learning about The GIMP's SDK. Many freely available filters that have been created for Filter Factory can also be used with this plug-in.
Arachne is program that implements Avidan and Shamir's seam carving technique for content-aware image resizing. It allows realtime, fluid image resizing using this fascinating technique. Seam carving works by removing the "boring bits" from an image, so it can change the aspect ratio of many images without changing the shape of the objects inside.
ingimp (Instrumented GIMP) is an instrumented version of the GNU Image Manipulation Program that automatically collects usability (usage) data, such as the commands used, the size of images worked on, and so on. This data is transmitted to ingimp.org where it is analyzed and available for anyone to download. ingimp is intended to shed light onto how the software is actually used in practice. It can be used as a snap-in replacement for the GIMP.
FrAid (Fr[actal] Aid) is a programming language that is appropriate for mathematical computations, visualization, batch processing, and more. It features both a standalone application that provides a programming environment for the language and a system for integrating the language with Java. The Java interface allows mathematical equations and formulas to be used with Java code, making it easier to process numerical computations, symbolic computations, imaging, CAD, and more.
Tux Paint is a simple and entertaining drawing program geared towards young children. It has a simple interface, sound effects, and a cartoon character (Tux, the Linux penguin). Along with drawing brush strokes, lines and shapes, you can also enter text and place "rubber stamp" (or "sticker") images on the picture. Tux Paint is extensible, and could be useful in an educational environment (such as a grammar, elementary, or grade school). It's portable across numerous platforms, and runs well even on slower systems like the Pentium 133MHz.