I am looking for a site with online video tutorials for Ubuntu / Linux users at different levels. Something like lynda.com (no affiliation) for Ubuntu users.
Check these sites:
Also you can always find videos in Youtube about various topics in Ubuntu. As far as I know, there isn't any such site dedicated to teach you Ubuntu. (I may be wrong)
I would recommend Ubuntu screencast and OMG! Ubuntu channel on Youtube most.
You can also check this brainstorm idea about Ubuntu video tutorial.