Light: A very detailed article, in several sections.
Part 1: The basics
Part 2: Light direction
Part 3: Natural Light
Part 4: Artificial and indoor light
These articles are the basis of a book which is out in spring 2011. If you enjoy the tutorial then the book offers an extra 11 chapters of information, covering everything from shadows, reflections, colour and finally how to use lighting for staging and storytelling. You can buy it from Amazon via the links at the bottom of the tutorials.
Digital inking: How to ink digitally and get good results (a little out of date now, but still useful)
Good Wacom technique: How to avoid the shakes when drawing with the tablet
One point perspective trick: How I did the floorboards in Rufus and Rex
Three point perspective trick: Block models as a drawing aid
Lighting in passes: A 3D technique applied to 2D.
The Monster Makeover: A Photoshop painting tutorial, featuring overlay layers