The Open Material Library is a new Blender material repository and even though it's still in beta (in good web 2.0 fashion), I think we may have a winner here!
At the moment of writing, the site contains 82 materials and it looks like it's growing rapidly. Browsing the site is a joy - each material is displayed with a preview image (a sphere, Suzanne or 'complex'), a clear description, a rating and room for user comments.
After registering you can add new materials to the library. You'll have to use the provided .blend file and render a few preview images. After the moderators are done with it, your material will be available to the rest of the community.
I have only one remark: they should add automatic rendering of the preview images. You can run Blender from the command-line just fine, so it should be easy to let the website's backend render those automatically (properly sandboxed for security, of course). Dropping the requirement to provide preview images will make it even easier to upload new material and I'm sure this would contribute in making this site a success!