A tutorial (MtoA) that covers how to render a polymesh as a volume can be found here.
An Arnold Material Library with over one hundred materials is available for Maya here.
Arnold Answers is a reputation-based forum where Arnold users of all kinds can ask their colleagues and peers questions related to Arnold.
It can also be found under Community, on SolidAngle.com.
The latest Maya 2018/Arnold 5.0 assets for the Autodesk internal production Hyperspace Madness have now been made available.
More Arnold renders can be found here.
A simple tutorial that demonstrates how to create a toon shading effect using the Facing Ratio shader can be found here.