A new Blender materials library is now available, and it includes an add-on (alpha version still) for easy access. Pretty cool!
Sergey Ozerov writes:
I'm happy to present you my project Blendermada - http://blendermada.com. Goals of this project are to make a good online materials database for most popular render engines for Blender and to extend this database with add-on, which can work with database directly from Blender.
Blendermada add-on still is alpha version and works only with Cycles, but it works properly and you can use it. All materials in library are published under CC-0 license and you can use it as you want.
Database is growing, but not so fast as I (and maybe you) wish. And if you can help me, I'll be happy. Especially if you can work with another engines.
Client and server are published on GitHub. If you find bug or issue, or just want to join development process, you can find it there.