GIMP: Free Image editor

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. 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.
GIMP can be downloaded from the following location.
It is available for the following Operating Systems: Windows, Unix, MacOSX

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