Retouch, convert, create images with layers and channels, using GIMP advanced photo editor
GIMP is a freely distributed program 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, an image format converter, etc. Note that GIMP won’t open RAW files, unless you install UFRaw or a similar plugin.
Features : Full suite of painting tools including Brush, Pencil, Airbrush, Clone, etc.; powerful gradient editor and blend tool; full alpha channel support; Layers and channels; Multiple Undo/Redo (limited only by diskspace);
Editable text layers; Transformation tools including rotate, scale, shear and flip; Foreground extraction tool; Advanced path tool doing bezier and polygonal selections; Transformable paths, transformable selections; Quickmask to paint a selection; Plug-ins which allow for the easy addition of new file formats and new effect filters; Over 100 plug-ins already available – Animation; Load, display, convert, save to many file formats, and much more.
Although a full featured painting program, GIMP opens faster than photoshop, however, if you need something lighter and simple, check PhotoFiltre.