Arnold 5.0 is now available. Please read the FAQ (MtoA) for details about transitioning to Arnold 5. More information here.

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.