A while ago i found a website much like jsFiddle, focused on collaborative editing. It has forking and it's full of HTML5 experiments. Unfortunately i can't remember the name; do you by any chance know which website it is?
EDIT It's not jsbin.
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For Flash, there are these two sites, both related:
More canvas-related is Aza Raskin's http://azarask.in/projects/algorithm-ink
More JS-related: http://jsdo.it/