Gimp

The GIMP is a multi-platform photo manipulation tool. GIMP is an acronym for GNU Image Manipulation Program. The GIMP is suitable for a variety of image manipulation tasks, including photo retouching, image composition, and image construction.

It has many capabilities. 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.

Full suite of painting tools including brushes, a pencil, an airbrush, cloning, etc.
Tile-based memory management so image size is limited only by available disk space
Sub-pixel sampling for all paint tools for high-quality anti-aliasing
Full Alpha channel support
Layers and channels
A procedural database for calling internal GIMP functions from external programs, such as Script-Fu
Advanced scripting capabilities
Multiple undo/redo (limited only by disk space)
Transformation tools including rotate, scale, shear and flip
File formats supported include GIF, JPEG, PNG, XPM, TIFF, TGA, MPEG, PS, PDF, PCX, BMP and many others
Selection tools including rectangle, ellipse, free, fuzzy, bezier and intelligent
Plug-ins that allow for the easy addition of new file formats and new effect filters