Photoshop.

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..