Photo Editing (Free) - GIMP

GIMP (GNU Image Manipulation Program) is a photography editing software which is developed for free photo editing. The GIMP is available for operating system including Microsoft Windows, Mac OS X and Linux.  GIMP was started to develop in 1995 as a  project at the University of California, Berkeley.

Fantastic GIMP Free Photo Editing Software

GIMP is completely free and easy to use. The features includes photo enhancement and digital retouching.
Here are the features extract from GIMP official website:

• 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 disk space)
  • Editable text layers
  • Transformation tools including rotate, scale, shear and flip
  • Selection tools including rectangle, rounded rectangle, ellipse, free, fuzzy
  • Foreground extraction tool
  • 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 more.

The official GIMP website provides detail tutorial and user manual on how to use this software. There are many books available in bookstore. In my opinion the best book is " Beginning GIMP: From Novice to Professional  by Akkana Peck".  Below is a video on youtube with brief introduction to use GIMP:

Click this links for GIMP download and start to learn how to use this fantastic free photo editing software.