Photoshop plugins are add-on programs aimed at providing additional image effects or performing tasks that are impossible or hard to fulfil using Photoshop alone. Plugins can be opened from within Photoshop and act like mini-editors that modify the image.
Photoshop-compatible plugins fall into several main types: filter plugins 8bf, import plugins (also called 'acquisition') 8ba, export plugins 8be, file format plugins 8bi, and automation plugins 8ly. Also, there are selection plugins 8bs and parser plugins 8by, but obviously no one else than Adobe has ever created plugins of these types.
The most common type are filter plugins. They have the 8bf file format and usually supply special image effects.
Import/export plugins acquire or write image data from or to certain devices, file format plugins open and save exotic image formats, and automation plugins automate certain tasks in the manner of Photoshop Actions.
|1991||Adobe first introduces filters and support for third-party Photoshop-compatible plugins in Photoshop 2.0. The same year, Aldus presents Aldus Gallery Effects - a set of filters including Emboss, Mosaic, Charcoal and other effects. When Aldus and Adobe merge in 1996, Gallery Effects will be embedded into Photoshop.|
|1992||Kai Krause releases one of the most renowned plugins of the 1990s -- Kai's Power Tools (a.k.a. KPT). Many artists of the time consider it a must-have plugin set for Photoshop. It features several advanced warp and deformation effects, as well as support for bump maps and 3D graphics formats (in KPT SceneBuilder).|
|1994||Joe Ternasky releases Filter Factory, a plugin allowing users to create their own filters using an internal programming language resembling C and compile them as separate plugins. It uses programmable formulas to process the red, green and blue channels of each pixel of the image. However, the fact that it requires considerable programming skills is viewed by many as a serious drawback.|
|1994||Alien Skin Software, founded a year earlier, creates the first drop shadow filter for Photoshop. The same year, they also release the Black Box filter set, later renamed to Eye Candy, which becomes an all-time favorite among Photoshop users.|
|1997||Alex Hunter, inspired by KPT but dissatisfied with the limitations of the Filter Factory, presents FilterMeister -- "a 'bigger and better' Filter Factory". It is said to be much easier to use than Filter Factory, and many of today's free and commercial plugins are made in FilterMeister.|
|2007||Filter Forge Inc. brings procedural texturing to Photoshop by releasing Filter Forge, a plugin allowing users to build custom filters without any programming. In Filter Forge, filters are assembled in a visual node-based environment.|
Host applications or plugin hosts are graphics applications that are capable of running plugins. Many commercial graphics applications support Photoshop-compatible plugins — Photoshop, Paint Shop Pro, Photoshop Elements, PhotoImpact, Corel PhotoPaint, and Adobe Fireworks are the most renowned ones. There are several dozens more plugin hosts, including free editors like GIMP with certain add-ons and viewers like IrfanView.
Photoshop fully supports all available plugin types; certain hosts, like Photoshop Elements, support most of them, while the majority of hosts support filter plugins only and many of them don't even support all available filter plugins.
The support for plugins was more uniform up until 2002, when Adobe restricted access to the Photoshop SDK containing the specifications for Photoshop plugins, and made the developer license more prohibitive. Since then, developers of other image applications have had limited or no access to it anymore, so they can't support newer host features. Therefore, plugin developers face a dilemma: either support the new host features that appeared in Photoshop 7 and later versions, like the access to layers, and lose the compatibility with other image applications, or use the old SDK version which already includes all important specifications and make sure the plugin will be supported by all hosts. 
Around 2005, Adobe changed the policy so that developers could make the request for the SDK via a Web form with no fee charged for it and with all requests handled individually.
Running Photoshop Plugins
In most host applications, plugins can be accessed via a menu that is usually called 'Filters' or 'Effects'. Each plugin collection is displayed as a submenu. In Adobe Photoshop, pre-installed plugins come first in the list. Third-party plugins are placed at the bottom of the menu, below the separator.
Kai's Power Tools
Kai's Power Tools (now called Corel KPT Collection) are a set of filter plugins created by Kai Krause. The various versions of Kai's Power Tools -- KPT 3, 5, 6, and X sets -- are compilations of different filters. KPT Convolver is used to customize design blurring, edge-detection, focus and color embossing. KPT Projector, mainly duplicating Adobe Photoshop's Free Transform capabilities, can rotate the bitmap image in 3D space and tile the results, as well as animate the distortions. The KPT Goo filter produces a single frame freeform liquid distortion. KPT Materializer creates advanced surface textures based on bump maps. The KPT Gel filter uses various paint tools to synthesize photo-realistic 3D materials such as metals, liquids, or plastics. The Equalizer filter with its three modes is used for applying variations on sharpening effects. KPT SceneBuilder produces photorealistic 3D scenes by importing and rendering 3DS files and can work as a standalone 3D modeling tool. KPT SkyEffects is designed to simulate the interaction between the light from the sun or moon with as much as six atmospheric layers of haze, fog and cloud.
Pros: fast rendering of effects, unique assortment of filters, custom settings can be saved as presets.
Cons: nonstandard interface, limited preview sizes.
Alien Skin Eye Candy 5: Impact
Eye Candy 5: Impact is a collection of ten Photoshop filters which create chrome, brushed metal, glass, bevels, shadows, reflections and more. Brushed Metal simulates textured metal surfaces such as brushed aluminum and polished brass; Super Star generates a wide variety of shapes, including stars, flowers and gears; Backlight projects a light beam and spotlight effects from behind any selection, and so on. Most of the effects can be applied to a separate layer, which gives users more control over the effect in the host application. Eye Candy 5: Impact contains more than 200 presets.
Pros: easy-to-use interface, adjustable filters, 16-bit image support.
Cons: not compatible with older systems and host applications  
Flaming Pear SuperBladePro
SuperBladePro is a plugin that combines textures with bevels and mirrorlike reflections. It helps quickly create tarnish, iridescence, and glassiness effects. Its features include waterstains, moss, abrasion, spotlighting, dust, grit, embossing, fractal blotches, and smooth Gaussian bevels. SuperBladePro also includes text effects that can be applied to the text itself, creating a contour around the letters, or to shapes, solids and areas with feathered masks.
Pros: a large number of controls allow users to customize their textures and texture effects, including importing bump maps and 3D lights.
Cons: not possible to run the help file from the dialog; too many options on a single dialog; the icon buttons don't have any tooltips or written explanations.  
Filter Forge allows computer artists to build their own filters – seamless textures, visual effects, distortions, patterns, backgrounds, frames, and more. The key features of Filter Forge include a visual filter editor and a free online library of user-created filters.
Filter Forge comes in three editions: the Basic edition provides unlimited access to the filter library but cannot create filters. The Standard edition allows photographers and graphic artists to create their own filters. The Professional edition provides features useful in architectural visualization, 3D rendering and design. It includes support for bitmaps as large as 65,000 x 65,000 pixels, 16 & 32-bit image modes, and floating point-based file formats such as OpenEXR and PFM.
Pros: easy to use, powerful, scalable, large number of filters.
Cons: high price of the Professional version, learning curve for the Filter Editor.
Auto Fx DreamSuite Series
DreamSuite Series plugins include several dozens visual design effects for type, graphics and photographs. These effects can turn photos into puzzles, create colored artwork, add stylized borders to photos, and more. Pro features enable combination of different effects together using layers and masking.
Pros: quality output; detailed parameters for each effect.
Cons: the main window cannot be minimized; no undos; mediocre performance and high memory consumption.