Tamir Lousky presents a new add-on that allows you to randomly assign materials to object faces.
I just finished writing a new addon, which is sort of a follow up to Patrick Boelens' excellent BlenderCookie python tutorial on how to create a random material assigner which changes the active material on the selected object.
Instead of this, my addon assigns a random material to each face on the mesh, from the list of available materials for that object.
It's useful for creating variations and adding realism to repeating elements, such as the leaves created by the Ivy Generator or the Sapling Tree addons.
You can view a short video explaining how to install and use the addon below, and read more about it and on the code behind it on my blog.