A small collection of ‘fancy’ materials like ‘cracked planet’, ‘blue flame’ or ‘Final Fantasy Ghost’

Over 40 fabric and clothes materials.

A small library of Dirt materials.

Car Paints library with over 20 materials.

I have created a breakdown of the steps I used to achieve an old worn gold material. It describes and highlights some aspects of the luxrender material system.

Treatkor has posted a library of 64 materials on the BlenderArtists.org forums. They’re not organized in any way, but you might just find what you need.