Photoshop plugins (or plug-ins) are add-on programs aimed at providing additional image effects or performing tasks that are impossible or hard to fulfill using Adobe Photoshop alone. Plugins can be opened from within Photoshop and several other image editing programs (compatible with the appropriate Adobe specifications) 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 no one other than Adobe has ever created plugins of these types.
"Import/export plugins" acquire or write image data from or to certain devices, "file format plugins" open and save less common image formats (not inherently supported by Photoshop), and "automation plugins" automate certain tasks in the manner of Photoshop "actions" (macros).
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 little known products like Chasys Draw IES, free editors like GIMP (with certain add-ons) and viewers like IrfanView. Much support is limited to the Microsoft Windows platform and .8bf filter plugins.
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.
|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.|
|1994||Auto FX Software is founded. In the subsequent years they release a couple of much-noticed Photoshop plugins and automated effects software products. Photo/Graphic Edges is one of the common image enhancement tools used by graphic artists of the time.|
|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.|
|2008||YouSendIt Inc. enables delivering files across different users/computers from within Photoshop.|
Eye Candy 5: Impact is a collection of ten Photoshop filters which create effects such as chrome, brushed metal, glass, bevels, shadows and reflections. For example, Brushed Metal is meant to simulate textured metal surfaces such as brushed aluminum and polished brass; Super Star generates a variety of shapes including stars, flowers and gears; and Backlight projects a light beam and spotlight effects from behind a selection. Most effects can be applied to a separate layer, giving users control over the effect in a non-destructive manner within the host application. Eye Candy 5: Impact contains 200+ presets.
Photoshop Plugins from Auto FX Software have been around since 1994 and provide the digital artist with a wealth of creative visual image enhancement tools. From realistic atmospherics effects like lightning, snow, rain, haze and fog to creative effects that turn photos into puzzles, create lighting and shading, adjust focus or add stylized borders to hundreds of new high resolution backgrounds and textures - there are countless visual options the user can shoose from to give their images that distinctive look and feel. The included presets make it easy to apply ready-made effect settings with a single click.
Auto FX Software recently upgraded their whole line of products. Their latest release at the time of this writing carries the "Gen1" addendum in the product name associated with the following updates:
• Newly Designed and User Friendly Interface • Newly Added Content and Effects • Limitless Image Size Capabilities
Pros: quality output; detailed parameters and presets for each effect; free trial available for download
Cons: no undos; high memory consumption. 
Filter Forge allows its users 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.
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. (KPT SceneBuilder and KPT SkyEffects are not available in the Corel KPT Collection.)
Since the introduction, this plugin received a noticeable interest from community of digital artists. And counts more than 12000 downloads of free version. It's an extension non-modal panel for Photoshop for digital painters and artists. It includes simplified instruments for color picking in various forms, including a color wheel intended for digital painting on the tablet.
Pros: non-modal panel for color picking, that opens itself without a click; introduces color wheel in form of a panel for earlier versions of Photoshop (CS4 and CS3) and CS5
Cons: in current version (1.1) for Photoshop CS3 the main window cannot be docked