The G'MIC plug-in for GIMP proposes a set of various filters to apply on your images, including artistic effects, image denoising and enhancement algorithms, 3D renderers, etc.. It is a quite large plug-in, integrating a lot of different effects.
dechroma is a simple but effective chroma noise reduction tool. Chroma noise appears usually in high ISO photographs. The method used in this script allows to reduce noise but does not affect details. It decomposes the picture in the LAB channels, blurs the chroma noise on the red and blue channels, and recomposes the picture. The default settings of the blur radius should be appropriate for general uses, but can be change. It's probably safer to apply primarily dechroma on selections to avoid some red or blue smears on transitions.