Brendon Murphy [Meta-Androcto] has released a 'meta material library' in which he has collected over 480 materials and textures and he's planning for 160 more. It is a monster collection of several material libraries that have been released so far and it's well worth checking out.
Brendon is still looking for more additions such as in the 'Special Effects' category. If you have something to share, be sure to check out this thread on BlenderArtists.org.
Also note, that Brendon is doing the proper thing by giving credit to everyone who's work is included. He's still searching for some contributors, so if you spot something that's yours, get in touch with him and it'll be fixed.
The only crit I can think of is that there's no easy to use index yet. Even though Brendon has included several scenes that line up the different materials, a collection of renders with the material names would be a great help to select the material that you need! (Of course, if would be even cooler if Blender had an option to browse materials by preview ;-)