Adobe's Plugins for a long time saved the company's bacon. When Photoshop had only so so masking tools compared to other Graphics Editors Extensis MaskTools really filled the gap. Likewise the lack of a really good set of natural media brushes until Photoshop 7 was filled by Right Hemisphere's Deep Paint. In sum, the Adobe plugins standard, which most major graphic editors support (in fact you can use this functionality in a graphics program along with full color model support as a criteria for an app being a serious graphic editing tool), provides welcome additional functionality to many graphics, video, and animation editing tools.

Adobe's List of Plugins - a master list

Next a list of plugin vendors used and reviewed here at thePhotoFinishes.com with Info resources:

AlienSkin - Image Doctor and BlowUpand Eye Candy 4000 are two top plugins from this shop.
Andromeda - has dozens of filters and some good bundles from time to time.
AutoFx - a wide range of plugins and bundles from Dream Suite to Auto Eye
Corel - got into the plugin business through some astute acquisitions.
>Alchemy - is my favorite filter buried in Corel PhotoPaint for dozens of customizable effects.
>Knockout 2 - makes very precise complex masking easy to do.
>Kai's Power Tools - one of the original and most inventive plugin makers; stunning effects.
Extensis - has sold its plugins to onOne Software and is now in Graphics database/portfolios
FlamingPear - Tesselation, SuperBladePro and Mr.Contrast are 3 of the hot plugins here.
Fo2Pix - the Buzz and Artmaster set of plugins simplify and highlight images in novel ways.
FrischLuft - FrischLuft means Fresh Air which applies to these shape and chroma plugins.
Human Software - provides popular Squizz and Textissimo among 2 dozen plugins.
Kinnetix Fx iShadow - translucent drop shadows, very sophisticated
onOneSoftware - has collected together a
PowerXchange - Photoshop plugins plus lots of other graphics extensions.
Redfield Plugins - Jama 3D, Lattice Composer, and Plasteroids are a few nifty plugins.
Right Hemisphere - Deep Paint, Deep Publish and other 3D visualization plugins.
Stoik - PictureMan, like Deep Paint, is standalone or very worthy retouch plugin.
The Plugin Site - has its own Galaxy Effects plus resources for dozens of others.
Ulead - this graphics editor vendor has a neat plugin and Java applet/plugin set.
Vertigo 3D - supplies 5 3D effects/plugins for Photoshop and Illustrator
Visual Infinity - Grain Surgery is a plugin which sharpens images smartly.
Xaos Tools - Typecaster, Segmation, Terrazzo, Paint Alchemy are key plugins.

Plugin list sites:

Photoshop Plugins - one of the best sites for Photoshop Plugins commercial plugins
Free Photoshop - good reviews and pointers to plugins
PhotoshopSupport - some very good plugins mixed in with hard sell
Free Photoshop - over 70 links but some of the links are out of date
Plugins Site - Lots of plugins info, reviews, and lists