Here's a resource you may know about already, but still definitely worth a mention! The Blender Open Material Repository is a site with over 200 native Blender materials available for download. Subjects range from typical (Car Paint) to the downright bizarre (Brains).
We are bringing up this site now because a number of improvements have been made or planned recently: the site has moved to its own top-level domain, www.blender-materials.org, and the test version of a plugin to download the materials directly to Blender is now available for test. Thanks to Christian "Paratron" Engel for this fantastic community resource.