GIMP Plugin Registry

The G'MIC plug-in for GIMP defines a set of various filters, including artistic filters, image denoising and enhancement, 3D renderers, etc..  read more »

GREYCstoration is a noise reduction plug-in.
It allows to remove noise (grain) or small artefacts (JPEG blocks effect) from color images.  read more »

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.