I would recommend Siger CMPP, not only it has hundreds of materials but the library-system is just awesome.
I have specific materials for different clients and I have made library folders for each client, then when I get
something to render I just open CMPP and drag´n´drop materials.
Easy as 1-2-3.
It can save anything from the 3dsmax material library material slot.
Also if you import a lot of materials from your scene to the CMPP user library, you render previews quite easily.
I use my farm computers to make the previews.
I bought it because of the materials but I would say the user library is the one I use the most.