In this .blend file I have created a blendgroup node that calculates the conductive fresnel terms in cycles, that includes absorption. This group has then been used to calculate the reflectance for approximate red, green and blue light wavelengths based on recorded data and the incoming ray angle. The file does this for 23 different metals and substances including Gold, Silver, Copper, Iron, Nickel, Zinc, Iridium, Osmium, Aluminium and Chromium. It actually worked much better than I was expecting and gave reasonable results for most of the metals, although a few look a little off. It's questionable whether this physically based method is much better than a coloured glossy shader. I have also included a fake brushed metal effect.

Note: The conductive fresnel term actually already exists in the cycles source code although there's no node for it and it does not seem to be used anywhere(I may be wrong about this). Perhaps in the future it will be incorporated and the complex nodegroup can be removed.