LuxRender has started a Materials Database from which you can download materials as well as upload you own content to it. It looks pretty impressive so far. While you are at the LuxRender site check out the gallery too. There are some wonderful Blender LuxRenders in the exhibit.

An library of odd and ends: stuff like electric bolts, baseball, cigarette filter and lollipop.

A collection of ‘halo’ materials.

A small collection of ‘fancy’ materials like ‘cracked planet’, ‘blue flame’ or ‘Final Fantasy Ghost’