Techzilo has a great list of the best GIMP Plugins available.

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.

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