A ton of video tutorials on sketching, marker rendering, and a few 3d topics are available on this page from MIT.edu.
WelcomeIDBoost is my own experiment to try to catalog some of the skills-based learning resources for designers I have found online. I'll post bits of info as I find or remember them. For more info on me, my design work, or my Gnomon Workshop DVD, visit www.speckofdustin.com. -Dustin Brown