In particular that run natively on Linux.
There are several different plugins for gimp to remove noise.
GREYC's Magic Image Converter (G'MIC) is a continuation of GREYCStoration. I'm not a fan of it, as I've found it to be very slow and there are loads of options to tweak. This means you could spend hours just tweaking the parameters to find an optimal set for a particular image.
My personal recommendation would be Wavelet Denoise. It's fast, has few parameters and generally produces excellent results. The results are not quite as good as the commercial packages, but they're close enough that you won't notice the difference unless you're pixel peeping. For best results, I use the YCrCb colour space and apply noise reduction to the Cr and Cb channel. Go very easy on the Y channel (the luminance channel), because if you over do it you'll end up with a very "plastic" looking image. I normally leave the Y channel alone as I like a little bit of grain in my images.
I can tell you a very simple trick that you can do it with any photo editing tool.
1) Double your image size,
2) blur it a bit,
3) bring it back to the normal size.