Blendermada is Blender Material Database.
You can find here a lot of materials, download them absolutely freely and even get an addon to import anyone directly to Blender.
Blender supports different render engines and Blendermada supports materials for these engines: Cycles, Blender Internal and others. Just select needed engine in search form at the bottom of material page.
If you want just download a material, you need no registration - just press download and save .blend file with material. If you want to join this project as contributor of vote for material, you should register.
Database wouldn't be good if you couldn't browse and import materials straight into Blender. But you can. We made special add-on for Blender and you can work with Blendermada inside your favorite 3D editor. But don't forget to visit http://blendermada.com sometimes, it contains much more information.