GIMP is the undisputed king of image editing in Linux platforms, and is next only to Photoshop in popularity in Windows and Mac platforms. With a large community of developers and an even larger pool of users, it is no surprise that GIMP is very popular.
Much like Firefox, GIMP’s strength lies in its plugins, which are developed by the open-source toting community. Since the users themselves develop them, they know all the needs and conceive a plugin for everything (well, except making a coffee for you 
).
And what better way for me to endorse GIMP than a list of good plugins? Most of them are aimed at web-designers and photographers, the main users of GIMP. There are also some to fix common problems of GIMP.
Web Design
- Snow: Generate realistic 3D snow. Caution: Requires large amount of memory and swap memory.
- Anti-aliasing: Smoothen and fix straight lines.
- Adaptive contrast enhancement: Intelligent adjustment of contrast.
- Fourier Transformation: GIMP plugin version of Fourier Transformation.
- Refract/Reflect: Refraction effects to simulate falling water drops.
- ShapeIt: Bend text or image according to information in map layer.
- Borders: Add borders to your image.
- SoapBubble: Create bubbles. Contains two versions, for GIMP v2.2 and v2.4.
- Aqua Bou: Create web buttons in ‘Aqua Bou’ style easily.
- Chrominium: Create a chrome logo with glow and sparkles.
- Cloud Generation: For easy creation of clouds. Updated to work with GIMP 2.4, the latest version.
- Gothic Glow: GIMP’s version of Gothic Glow action, originally for Photoshop.
- Liquid Rescale: Content-aware rescaling. Keeps the features of the image while rescaling along a single direction.
- Particle Deposition: simulates particle deposition on a surface. It generates small particles with random size and velocity and lets them fall onto the surface where they settle, increasing surface height in the place a bit.
- Stampify: Make your image look like a postage stamp.
- Plasma2: Newer version of the original Plasma plugin for GIMP.
- SaveForWeb: Find optimal compromise between minimal file size and acceptable quality of image quickly. Useful for designers.
- Steel Text: Create a text effect that looks like shiny steel (or gold) and a dropshadow.
Photography
- Pandora: Stitch together multiple images to make a panorama.
- Film Grain: Add film grain for a classic feel, to BW images.
- Black Ink: Transform an image to a B&W drawing made with ink.
- Copyright Text: Add a copyright text to image.
- DeNoise: Noise reduction plugin using a modified Gaussian blur algorithm.
- DustCleaner: Detect and remove the dust spots in digital image.
- Astro-plugins: A set of plugins for astronomical image processing.
- FocusBlur: Makes an image out of focus with luminosity and depth.
- Lomo: Make photos clear, sharp and crisp. Especially useful for outdoor photographs.
- ISO Noise Reduction: Implements an ISO noise reduction, using different methods (masking the edges, then blurring the individual color channels or the lumimance channel only).
- Photo Effects: A bunch of scripts brought together in one script-fu file. Contains cartoon, color pencil, conte crayon, cutout, Drawing, ink pen, note paper, paint dot image, palette knife, pastel, stamp, water paint effect, wrap effect, angled strokes, crosshatched, chrome image, cross light, funky color, soft focus, solarisation, brick wall, patchwork, stained glass, texturizer, high pass, scroll effects.
- RedEye: Quickly remove red eye effect caused by camera flashes.
- ShadowRecovery: Recover image information in darked regions.
- Shadows/Highlights: Eliminate shadows and highlights in photographs.
- Refocus: Opposite of FocusBlur(listed above), this refocuses images. Provides better results than Unsharp masking which is built into GIMP.
- PhotoToolbox: Perform several actions on a photo at once such as defocus, desaturate (several papers emulations), toning, add grain.
- Light Filter: Provides basic filtering by adjusting colour levels of each pixels.
- Gradient Along Path: Strikes a vector applying a gradient orthogonally to the path.
- Landscape painter: Creates interesting paint effect that looks best with landscapes.
- EZ Improviser: Improve dull, dingy images.
- Edgy: Make an image look edgy.
- Digital IR: Simulate a black and white infrared image.
- Colour saturation: Change the color saturation of your image.
- Blot: GIMP avatar of BlotBrush. Based on blot algorithm and works well with spatter or cloudy image.
- GimpPublishr: Publish images to Picassa and Flickr directly from inside GIMP.
Miscellaneous
- GIMP Dewierdifyer: (for Windows) Get rid of common window annoyances in the GIMP. It adds a background window that sits under GIMP. It groups other GIMP windows into one, so that when you’re alt+tabbing to other apps, only one icon of GIMP gets in your way.
- Turing Pattern: Renders a kind of Turing pattern.
- Yin-Yang+: A seemingly useless plugin, but fun nonetheless. Add yin and yang symbols. Also optionally add drop-shadows.
Further resources
- Writing a GIMP plugin: Plugin guide in PDF, HTML, archive and XML formats.
- GIMP plugin template: A blank plugin template that has the basic structure and reduces the time required to develop a plugin considerably. The direct link is currently here, but may change in future (if it changes, click the link on “GIMP plugin template”.
- Writing plugins: Another guide for writing plugins, this one from GIMP.org’s developer site.
- Writing Plugin: Another guide at GIMP.org main site.