GIMP Plugin Registry

Implements an ISO noise reduction as described in the tutorial below, also this script uses different methods (masking the edges and then blurring the individual color channels or the luminance channel only)

Tutorial: http://www.gimpguru.org/Tutorials/ReducingCCDNoise/

PS - Using a plugin like Wavelet denoise from http://registry.gimp.org/node/4235/ or greycstoration from http://www.greyc.ensicaen.fr/~dtschump/greycstoration/ might actually give you better results and more options for fine tuning.