Some of these have probably been posted before but Here’s a few goodies.
http://archive.cardesignnews.com/studio/tutorials/index.php
http://www.lugnegarddesign.com/
http://massiveblack.com/mbNew/html/concept8.shtml
http://www.drawthrough.com/galleries.php?gallery_cat=id
http://www.michaelditullo.com/Design/JORDAN.html
http://www.coroflot.com/public/individual_set.asp?from_url=true&sort_by=1&set_id=23781&individual_id=86739
http://www.toyfon.com/html/design_concepts.htm
http://www.simkom.com/sketchsite/
http://www.local-motors.com/designwall.php
http://www.idsketching.com/
http://www.coroflot.com/public/individual_set.asp?individual_id=15470&set_id=308190&
http://www.coroflot.com/public/individual_file.asp?individual_id=220988&portfolio_id=1618770&is_featured=-1&
http://oneliterracing.blogspot.com/
Enjoy!