Indispensable when you shoot in low-light conditions. You can do it in Photoshop with filter and some manipulations. I have found it easier (can’t say better, since I have not done any tests) to use a dedicated sw to do it. (BTW, if you are going to use noise reduction, you should do it at the very beginning of the development process.)
Noise Ninja
Picturecode
www.picturecode.com
Exists in a stand-alone version and as a plug-in to Photoshop. I like the stand-alone better, and it has a batch function that works well. User instructions aren’t always obvious though. It turned out (if I understood things correctly) that the sw can not choose a built in noise profile based on EXIF from a tiff file (but you can do it with jpg I believe) even though you are given the impression it works. I resorted to always letting the program auto-generate a noise profile. Works pretty well.