The GIMP image editing software can be used for photo retouching, image composition and image authoring.

About GIMP: The GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software that can be used as a simple paint program, an expert photo retouching program, an image format converter, as well as a range of batch processing and rendering operations.

Runs on:  UNIX, MS Windows and Mac OS X.