Alien Skin. This respected company makes great plugins for huge photo enlargements, really difficult image fixes, and just plain fun stuff.
Andromeda Software. This long-established company offers dozens of plugins. Right now there’s a great deal going with a bundled set that features Andromeda, Alien Skin, Digital Element and Nik Multimedia plugins.
Auto FX Software features elegant plugins and stand-alone software for visual applications.
Daily Textures. Not a plug in, but a great source of high-resolution textures, some subtle and some dramatic. Some vaguely allude to seascapes, dark forests or stormy skies. Great fun.
Flypaper Textures. This isn’t a plug in, but rather a series of high-resolution textures that can be used as overlays for some truly spectacular as well as subtle effects. There are free textures out there, but you get what you pay for.
Nik Software, recently acquired by Google, is an excellent source of high-quality plugins. The bundled set mentioned above is also available on this site.
onOne Software offers all kinds of amazing tools, from black and white, to upsizing and masking.
The Plugin Site has both free and for-purchase plugins, plus reviews, resources, discussion and more.
Topaz Labs. This company offers a vast array of photo enhancement and correction tools. Some truly amazing plugins for special effects and retouching. I often use Topaz Impressions for some great painterly effects. Free 30-day trials available.