Here is a list of some adobe photoshop plug-ins that are very good. Please feel free to add suggestions.

LIST:
Vignette Remover- removes vingnetting
GradientXterminator
IntelliSharpen II - Sharpening
Velvia Vision - Velvia Film like effects, good for landscapes in particular
Dynamic Range Increase - Quick way of increasing DR
SI Pro 2 - Interpolation/resizing
PT lens - for removing lens distortion
OptikVerve- Various virtual filters and effects (free)
Noise Ninja- Remove's noise from images (a must for all photographers)

Other Sites:
http://www.thepluginsite.com/
http://www.freephotoshop.com/html/plugins_types.html