Some incredible work by Yuki Yoshida using Arnold for Maya. More examples on his website.

Zeno Pelgrims has created a nice dust shader for Arnold for Maya (MtoA).

Oliver Couston has created a great guide, showing the workflow from Substance Painter to Arnold.

I got some interesting results connecting texture maps to the UV Coords of an Ai Image shader recently.

A tutorial that shows the basic workflow can be found here.

Arnold Kicks Ass

Zeno Pelgrims has created a shading community website that will host monthly material challenges, and will be used as a place to talk about shading/renderers/algorithms in general.

www.shaders.xyz

Mostafa Azab has a free Standard shader pack available for C4DtoA.

Maximeroz has an excellent HDRI pack available for free.