GE - Imagination Cubed - A lovely big clean page to draw on with a marker - though it lacks an eraser. You can also use IM or email to invite friends to join in.
Youdraw.com - offer a simple interface which some people do amazing artwork on. You can submit your drawings for display. Youdraw have even published posters filled with hundreds of drawings, and a book is in the works.
Graffiti.playdo.com - is an online graffiti board, but you are drawing with strangers, most of whom seem to just want to scribble over everything! Beware of bad language, too.
mydrawings.com is a collective drawing / graffiti space, where you can vote on other people's artwork. It also has a mix of interesting, creative stuff and inane drivel, and again, watch out for the anti-social behavior.
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