Gimp is an free image editor program. 
The supported platforms are:
GNU/Linux (i386, PPC)
Microsoft Windows (XP, Vista)
Mac OS X
Sun OpenSolaris
FreeBSD

Features and Capabilities
Painting
Full suite of painting tools including Brush, Pencil, Airbrush, Clone, etc.
Sub-pixel sampling for all paint tools for high quality anti-aliasing
Extremely powerful gradient editor and blend tool
Supports custom brushes and patterns
System
Tile based memory management so image size is limited only by available disk space
Virtually unlimited number of images open at one time
Advanced Manipulation
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
Selection tools including rectangle, ellipse, free, fuzzy and intelligent
Advanced path tool doing bezier and polygonal selections.
Transformable paths, transformable selections.
Quickmask to paint a selection.
Extensible
A Procedural Database for calling internal GIMP functions from external programs as in Script-fu
Advanced scripting capabilities (Scheme, Python, Perl)
Plug-ins which allow for the easy addition of new file formats and new effect filters
Over 100 plug-ins already available
Animation
Load and save animations in a convenient frame-as-layer format
MNG support
Frame Navigator (in GAP, the GIMP Animation Package)
Onion Skin (in GAP, the GIMP Animation Package)
Bluebox (in GAP, the GIMP Animation Package)
File Handling
File formats supported include bmp, gif, jpeg, mng, pcx, pdf, png, ps, psd, svg, tiff, tga, xpm, and many others
Load, display, convert, save to many file formats
SVG path import/export
Much, much more!
Tutorial Link