Script-Fu -> ISO Noise Reduction
This is an adjustable script,
it acts a lot like a softening filter, but tries hard not to over-soften edges and detail.
ISO Noise describes a problem in digital photography: you can get highly contrasting color specs scattered over an image -- some spots just too light or dark... some too saturated...
Basically -- you can't shoot a digital cameral a 400 ISO and expect it to come out looking like a Kodachrome slide. This filter helps.