The main ones are the very comprehensive 'Interactive Training Guides' (ITG) provided by Graphisoft -
http://www.graphisoft.com/products/archicad/training_guides/
There are also a few tutorials listed in the 'Tips & tricks' section of the forum -
http://archicad-talk.graphisoft.com/viewforum.php?f=8
The tips & tricks articles are generally kept on
http://www.archicadwiki.com/
but there seems to be no easy way of finding them unless you know exactly what to search for!
Good luck!