A simple freeware program for creative painting. Includes a highly configurable brush system and built-in teamwork features.

A semi-professional, open source image processing software suite that includes layer support and a wide range of correction tools and filters. A user guide is available here.

The portable version of the semi-professional, open source GIMP image processing software suite that includes layer support and a wide range of correction tools and filters. Runs directly from USB or other memory media and requires no additional installation. A user guide is available here and you can download the source code here.

A plug-in for GIMP that contains a large number of predefined image filters and effects. The source code is available for download here.

Please note: The contents of the archive have to be copied to the GIMP plug-ins folder, which can usually be found under C:\Programs\GIMP-2.x\lib\gimp\2.0\plug-ins\. The G'MIC entry will be listed in the GIMP Filter menu when you restart the program.

A comprehensive image and icon editing software package. Requires Java Runtime Environment (JRE) 5.0 or higher to run.

A comprehensive suite of tools for stitching horizontal and vertical panoramas. Automatically generates control points that can be manually adjusted by the user. The source code is available for download here. Requires .NET Framework 3.5 or later.

An open source vector graphics editor with capabilities similar to Illustrator, CorelDraw, or Xara X. Uses the W3C standard scalable Vector Graphics (SVG) file format. Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and makes it easy to edit nodes, perform complex path operations, trace bitmaps and much more. The source code is available for download here.

Luminance HDR is a graphical user interface (based on Trolltech's Qt4) that provides a workflow for HDR imaging. Supports HDR, TIFF, PFS or EXR, as well as JPEG, PNG, PPM, PBM and TIFF LDR images. You can download the source code here.