I see quite a few interesting questions on Stack Overflow where some basic interactive demos could add to the text/code explanations.
I imagine for questions like this, having the option to mark code as an
'Active Code Sample'
and run it through ProcessingJS would be fun.
On ActionScript related questions, I admit this might be stretch, since a lot of people disable Flash by default and some hate it, but maybe embedding demos written on wonderfl might help questions/answers.
Just a thought.