Problem is, there are quite literally thousands
of plug-ins, ranging from very user-friendly to those with rather steep learning curves, from cheap (or free) to absurdly expensive, from very general-purpose to extremely specialized, from very effective to somewhat worthless, from... etc, etc. The way you laid out your question, this thread might end up being just another chaotic list of existing plug-ins (and there are already much more organized ones, such as this one
or this one
), or something of a poll on plug-in popularity (which, in and on itself, is pretty much as useless, since popularity is nowhere near the best indication of actual value).
If what you need is to quickly find out which are (or might be) the best tools for a specific processing task, or at most two or three specific tasks, I suggest that you ask for those specific tasks, so as to focus the discussion in the areas in which you're specifically interested, lest you end up being subjected to a lot of information and recommendations about some image processing tasks that perhaps you weren't interested in at all.
For example, if you think you need to find out about sharpening plug-ins, then the discussion might purposefully concentrate on mentioning and comparing the characteristics of several well-know tools such as Focus Magic, Nik Sharpener, FocalBlade, Magic Sharpener, FocusFixer, etc, etc., and from that discussion, you'd probably be able to quickly choose the one that best suited you
A forum search might help, too, since many issues are bound to have been discussed already at lenght.