Here's the ones I use:
Neat Image for noise/grain reduction. Noise Ninja delivers similar results, but I don't see enough benefit to make it worth owning both. You'd probably be happy with either one if you take the time to learn how to use them to their fullest.
Focus Magic for what is commonly called "capture sharpening"--reversing the effect of the camera's AA filter. It works well in concert with USM. USM does not actually sharpen, it increases contrast. Focus Magic actually sharpens, but does not increase contrast. Using both together intelligently gives one the best of both worlds. I recommend using Focus Magic first, then USM next.
Midtone Sharpening Action Set Version 8.2 is not technically a plugin, but an action set that uses USM in concert with a mask that protects the highlights and shadows from being clipped. THis allows a greater degree of sharpening to be done without ruining the image with sharpening artifacts.
Convert To B&W Pro is what I use for all of my B&W conversions. The filter section is equivalent to the Channel Mixer method, but the color channel graphic EQ, contrast controls, and toning controls offer even more power and flexibility. IMO it's a must-have for any photographer who's serious about B&W. I regard it as one of the best $99 software investments I've ever made.
