Noiseware Community Edition is a handy free noise reduction program from Imagenomic. It's available for download on this page (the very last link). Unfortunately it only works on PC, which is disappointing for Mac users, but Imagenomic's other products are cross-platform.
The Community Edition is a standalone program, which means a bit of fiddling in your workflow, because you have to leave Photoshop, run the noise reduction, then return to Photoshop. If you do a lot of noise reduction, you'll probably find it worthwhile to pay for one of the Photoshop plug-in versions (available on the same page).
Noise reduction can be done at any (chronological) point in a non-destructive workflow - you can do it first, or last, or in between. What's important is that it's done on a layer immediately above the Background layer, underneath all of your adjustment layers.
It seems easy and logical to do your noise reduction at the beginning of the workflow, but that's not always practical, and sometimes it's not necessary at all! Don't forget that minor noise is likely to be invisible in print, so you needn't waste any time on it. I often consider noise reduction as part of my output workflow, when I know the exact print size required, and I can decide whether to do it or not.
But for this tutorial, I will discuss using Noiseware as the very first step in the Photoshop workflow.
Step One (for Raw files)
If you are working with a Raw file, make sure you begin by utilising the noise reduction sliders in your raw processor. In Adobe Camera Raw, they are available under the Detail tab (found just below the histogram). ACR's noise reduction is not the world's best, but it can handle mild to moderate noise quite nicely. Remember to zoom in to 100% when handling noise, so you can clearly see the effect of your adjustments.
After you open your raw image in Photoshop, you might decide that more noise reduction is needed. If so, choose File > Save As, and save your image as a jpeg. Make sure you choose to save it as copy:
Don't close the image you have open in Photoshop, just minimise Photoshop for a moment, because you'll come back to it soon.
or, Step One (for Jpeg files)
If you open a Jpeg file in Photoshop and decide it needs noise reduction, obviously you don't need to save it as a different jpeg just to run NR on it. Just mimimise Photoshop for now.
Open the Noiseware Community Edition program, then click "Open" and find the jpeg file you want to work on. (Remember, if you started with a Raw file, you've saved a jpeg copy, so open that. If you started with a Jpeg file, just open that file directly.)
Once it's open, make sure you zoom in to view the pixels at 100% size, by pressing the "Zoom In" button as many times as necessary. The zoom size is displayed at the bottom right hand corner of the interface.
The default setting is often pretty good, I find, so begin by simply pressing "Go". The noise reduction will run (takes a few seconds), then you can click anywhere in the image to compare the original to the noise-reduced result.
If you aren't satisfied with the default settings, choose one of the other settings and press "Go" again.
Generally speaking, I tend to choose a slightly too-strong setting, because I know I can reduce it a little in Photoshop.
Press the "Save As" button to save your image. The program will automatically save it as a copy of the original file, with the word "filtered" in the file name, so there's no risk of saving over another file.
Open the "filtered" jpeg in Photoshop. Now you should have two files open in Photoshop - the original photo that you opened and analysed before, and the new noise-reduced file.
Copy the filtered image and paste it onto the original image.
Ctrl A, Ctrl C, Ctrl W, Ctrl V
(This means: Select All, Copy, Close, Paste)
Then you'll have the filtered image on a new layer above the original image, like this:
If, like me, you've erred towards slightly too-strong noise reduction, you can reduce the opacity of the layer until you get a satisfactory result.
Remember to be viewing the image at 100% when you do this!
Go ahead and proceed with your edit!
Make sure you delete the "filtered" jpeg that you made. I can't think of a single reason you'd need to keep it, because you've already transferred those pixels into your master file. The last thing you want is unnecessary files cluttering up your system.