Maxwell Render from Next Limit Technology is an up and coming plug in for Max and Maya (Win 32 now and soon Linux and Mac OS X) that looks rather interesting. Rather than using a specific color space (think RGB) it uses spectral rendering technology. Check out the website and gallery images for a closer look at what it can do.
Richard also has some great images and brief overview of this technology on his website.
If you are looking for some great tips/tricks and general know how in using Maxwell check out the following from Metin.