For The Photographer who has no Post Production Software: And it's Free !

GIMP is a versatile graphics manipulation package. http://www.gimp.org/

Introduction to GIMP
GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.

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.

GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.

GIMP is written and developed under X11 on UNIX platforms. But basically the same code also runs on MS Windows and Mac OS X.

Features and Capabilities
This is only a very quickly thrown together list of GIMP features. You can also have a look at the illustrated features overview.

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, 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, much more!

GIMP is ideal for advanced photo retouching techniques. Get rid of unneeded details using the clone tool, or touch up minor details easily with the new healing tool. With the perspective clone tool, it's not difficult to clone objects with perspective in mind just as easily as with the orthogonal clone.

File Formats

The file format support ranges from the common likes of JPEG (JFIF), GIF, PNG, TIFF to special use formats such as the multi-resolution and multi-color-depth Windows icon files. The architecture allows to extend GIMP's format capabilities with a plug-in. You can find some rare format support in the GIMP plugin registry.

Supported Platforms
GNU/Linux (i386, PPC)
Microsoft Windows (XP, Vista)
Mac OS X
Sun OpenSolaris
FreeBSD

Digital Retouching
GIMP is ideal for advanced photo retouching techniques. Get rid of unneeded details using the clone tool, or touch up minor details easily with the new healing tool. With the perspective clone tool, it's not difficult to clone objects with perspective in mind just as easily as with the orthogonal clone.

Hardware Support
GIMP includes a very unique support for various input devices out of the box. Pressure and tilt sensitive tablets, but also a wide range of USB or MIDI controllers. You can bind often-used actions to device events such as rotating a USB wheel or moving a MIDI controller's slider. Change the size, angle or opacity of a brush while you paint, bind your favorite scripts to buttons. Speed up your workflow!

Customizable Interface

Each task requires a different environment and GIMP allows you to customize the view and behavior the way you like it. Starting from the widget theme, allowing you to change colors, widget spacings and icon sizes to custom tool sets in the toolbox. The interface is modulized into so called docks, allowing you to stack them into tabs or keep them open in their own window. Pressing the tab key will toggle them hidden.

GIMP features a great fullscreen mode allowing you to not only preview your artwork but also do editing work while using the most of your screen estate.

I can attest to GIMP being very functional and quite accomplished application on its own- I would definitely highly recommend it if you are looking for a post production image editing tool. Great recommendation k8mia .

how does it compare with photoshop???

Johnson wrote:

how does it compare with photoshop???

Certainly does not have the 100's of plugins that photoshop has, and has a slightly different way of doing things. I do use both, and will always use photoshop in preference as I am more familiar with it. I certainly don't use photoshop to it's fullest extent, and that may be where some of the features, enrichments may be differing the most. But as a tool to manipulate images and perform the above average editing capabilities for photos, it does well. There is a large user support which is great to ask questions if you are trying to achieve a task and are not sure on how. It is a relatively small Download and worth giving a go if you need these capabilities on another machine that does not have a PS license.

I hope others with more use can give feedback. I used to frequent another web group (www.3dbuzz.com) where they may be able to give a great reference for the differences (the site is a graphics training/learning 3D community)