I've been using Photoshop since version 4.5. Over the years I've tried oodles of plug-ins but there are a few that are now permanent parts of my work, and I find them extremely useful:
1) Focus Magic, which is a deconvolution program. It actually measures the width of the blur in your image and when the number-crunching is done, the image is re-focused. Not sharpened in the usual sense of the word, but re-focused. It will also help remove blur from camera movement. The re-focusing is subtle, and through masks or selections you can use it selectively. It is not capable of saving a truly out of focus image, but in my workflow it is used to counteract the A-A filter's softening. Usually it does such a good job that no further sharpening is neccessary unles I am printing larger than 11x17. When going larger I use Qimage's Smart Sharpening and Hybrid interpolation.
2) NIK Viveza and Dfine 2 are indispensible. Dfine 2 has replaced both Noise Ninja and Neat Image in my arsenal, and gives me as much if not more control with much, much less effort. Viveza is wonderful for local color and tone adjustments and for me, at least, is more accurate and easier to use than even the new adjustment brush in LR2.
3) NIK Silver Efex, which again gives me more control, and with much less effort, for converting to B&W than anything in LR or CS4. It has many more options, many presets that automatically show how your image would look, the ability to use the presets as a starting point and then alter any aspect of the image, and then to save new presets. The big advance in Photoshop is the B&W converer, either in RAW or as an adjustment layer, that gives you sliders for each color. Well, in Silver Efex that type of panel is just one tiny part of the available tools. You can also use control points to fine tune any area large or small to give exactly the tone and contrast you want.
4) NIK's Color Efex 3. This has some filters that can give very subtle effects, as well as others that make very creative starting points which are then easily adjustible.
5) I also keep Grasshopper Software's Image Align Pro on board. This gives more tools and control for fixing with ease the following parameters: Pincushion, Barrel Distortion, Rotation, Vertical Perspective, Horizontal Perspective, Vertical Skew, Horizontal Skew, and Scaling. There is nothing in Image Align Pro that you cant get to through Transform, Free Transform and Lens Correction but it is all in one place. You can also easily move the center of the activity and monitor the results immediately.
6) While not a plug-in, Qimage is also indispensable to me for printing. It has the following advantages: Better interpolation than CS4, at least for my images. Output sharpening that automatically takes into account the paper profile and size. Both are done on the fly so you don't have to save a huge file before printing. Also, I have found Qimage's softproof to be more accurate than Photoshop in terms of showing the actual areas that are out of gamut and how they will look in the print, rather than CS4's OOG warning, which puts gray splotches all over the image while Qimage shows much smaller areas but is much more accurate. Qimage also overrides the quirky insistence of my HP Z3100 on unequal margins, allowing me to center images on the print where Photoshop would not. Finally, Qimage allows me to set up picture packages with more ease and greater flexibility than CS4.
I do not work for NIK, Qimage or Focus Magic or the other publishers. But in all of my years with Photoshop, I can say that these plug-ins either give me functions I could not do in Photoshop, or allow me to do them more easily and quickly. Any way, I hope you find this useful. You can Google the other software. All allow free trials.