Graphic.com Has a nice article explaining three ways that you can use to reduce the noise from digital images. Anyone that has ever been involved with stock photography knows how hard this is. The main reason is that if you overdo it you introduce artifacts in the image which are even worse than noise imho.
The three ways that Graphic.com describes work (I usually use the first one) but I thought that I should mention a photoshop plugin that does exactly that reduce the digital noise. There ae several plugins of course but this one is free. It’s the DeNoise plugin.
Another free option is a standalone application, the Noiseware Community Edition (scroll to the bottom of the page). Although it has the disadvantage that it is a stand alone application it is really easy to use and provides quite good results.