Dave Johnson’s blog has very good material regarding mental ray, character technical direction and MEL:

http://www.djx.com.au/blog/

Zap Andersson is an engineer with mental images who worked on the fast skin material. His blog is a great resource for mental ray information:

http://mentalraytips.blogspot.com/

The Los Angeles mental ray User Group has all sorts of mentral ray-related goodies:

http://www.lamrug.org/

mymentalray.com is a growing resource for the mental ray community:

http://www.mymentalray.com/

A personal favorite is the page explaining sampling in mental ray, a good thing to understand for high-quality and efficient renderings:

http://www.lamrug.org/resources/samplestips.html

Eric Pavey’s MEL Wiki is an excellent aggregation of information for Maya’s built-in scripting language, MEL:

http://mayamel.tiddlyspot.com/

There’s always Duncan Brinsmead’s blog…recently showing some fun with Maya’s nCloth for non-cloth simulations:

http://area.autodesk.com/index.php/blogs_duncan/blog_list/

Here is a very good Maya Wiki:

http://www.tokeru.com/t

I will update this post as things change or as I come across additional resource.