Good free noise removal program for photos?

I took some pictures in the evening with my Nikon D3100, they look okay but the sky is noticeably grainy. I have both RAW and JPG versions of the photos, but am particularly interested in working with the RAWs since I heard that gives better results - I'm still kind of new to the whole DSLR photography thing. Are there any good freeware programs that can remove the noise? Must be Windows compatible, I'm running 7.
Any recommendations would be appreciated.

I'm not sure about any free ones that will reduce the noise. Demo Noise Ninja and see if you like the results. Really good things usually aren't free.