VIPS is a free image processing system.
Krita is a KDE program for sketching and painting, offering an end-to-end solution for creating digital painting files from scratch by masters, is available for Linux and Windows.
GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It works on many operating systems, in many languages.
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.
LightZone is professional-level digital darkroom software for Windows, Mac OS X, and Linux, that includes RAW processing and editing.
Piwigo is open source photo management software.
Fotowall is an opensource creative tool that lets you play with your pixels as you've always wanted! Make the perfect arrangement with your photos, add text, live video from your webcam and the best internet pictures.