In a recent post on CgCookie Kent Trammell explained that you can improve the quality of a Cycles render by rendering the image multiple times if you use a different value for the sampling seed.
In his case those images were calculated by different computers of his render farm.
But the same is true, if you waited for an hour for a render to finish, and you are pleased with the image except for the noise. In this case save the image, change the seed value and render it again. This way the first hour was not in vain.
In any case you will end up with one or more PNG files with look similar except for the noise pattern which changes with the seed value.
If you calculate the mean image of these images the noise level will be reduced.
Kent Trammell showed how this calculation can be achieved with the Blender node editor (his math was a bit off, but the principle is sound).
The same could be accomplished with other programs like Photoshop or The Gimp or command line tools like ImageMagick.
The ImageMagick command for doing this is:
convert image1 image2 image3 -evaluate-sequence mean result
However, if you try this with PNG files that contain an alpha channel (RGBA) the result is a totally transparent image.
In this case use the option "-alpha off", e.g. like this:
convert render*.png -alpha off -evaluate-sequence mean result.png
A final note: Keep in mind that all images will be loaded into memory during this process - i.e. you want to limit the number of images.