Besides the Art/Special Effects filters that I have a slew of, the ones you may get the most, actual utilitarian use out of could be :
Noise Ninja ... I'll second that one! If you severely crop or enlarge your shots; you're working with older, lower-quality images; you do night, low-light or other long-exposure shots; or you're in love with the high ISO settings - this software can be amazing...
Blow Up This is a little bit like Noise Ninja, but focuses more on making enlargements of images. Either for making HUGE prints (like wall-mural size), or because you only have a small, low-quality image to begin with. This can help "little" photos print better at larger sizes without fuzzing-out so bad - or make billboards out of high-res ones!
Nik Color Efex Pro 3 This set of filters simulates many of the traditional "screw-on" filters us old guys used to tote everywhere, and then some... Versions are available for either Photoshop or Capture NX 2.
Color Mechanic Pro This allows you to highly adjust individual colors in an image, and also makes it fairly easy to shift one color to another. (But if all you need to do is tweek-up the color a little, it's overkill. Use the Photoshop Adjustments or Camera Raw.)
As mentioned, I have a folder-full of plug-ins and filters, but those are the main ones I use for improving the quailty of photographic images (apply as needed). When I cut loose and paint, I like Virtual Painter 5 and a handful of other "art-style" plug-ins.