There is this that could be very close to a material library for Blender :
website :
http://shadertools.tuxfamily.org/
What ShaderTools?
* The Shader Tools is an add-on for Blender3D.
* It’s a extension which enable you to import from a predefined materials database and to include them in your projects.
* It can create and backup your own materials or external directly in the database.
<b>* It can also export your materials and you can to share them easily.</b>
That sounds rather interesting, not sure if there are plan to get this integrated into trunk at some point or if it will stay separated.