To start off the Mental Ray specific thread we will post some useful links. Mental Ray is a third party render engine by 'Mental Images'. It is a very mature application with a shader system about as advanced as you would ever want. It is the default render engine for SoftImage XSI and it comes bundled with Autodesk Max and Maya (I believe it is bundled with Maya not for certain). In years past artist struggled with MR due to all it's features and settings, more recently an intuitive interface has made it much easier for the average user and it is becoming a threat to the 'King of ArchVis - Vray'.
Mental Images - Mothership:
mental images - rendering imagination visible
LAMRUG - LA Mental Ray User Group:
LAmrUG.org
Jeff Patton - Mental Ray Expert:
JeffPatton.net
Mental Bout Max - Mental Ray Training:
Welcome to mental Bout Max - Video Training for the Mental Ray renderer in 3Ds Max
My Mental Ray - Mental Ray Community:
My Mental Ray
Zap's Mental Ray Tips - MR Blog:
zap's mental ray tips
That should start the ball rolling we can add to it as time allows and invite the community to share any good links they come accross.