I haven't touched Flash in awhile, but Actionscript guru Colin Moock's product Union Platform might be worth a look. I don't think it's open-source, but it is free up to 1000 concurrent users. And it's not just Flash or just chat -- more like a modular framework for building real time communication apps -- but Flash chat is clearly a use case.
I haven't used it, just heard about it. But Moock is a pro. And the product looks pretty flexible, powerful:
On Mar 13, 2013, at 5:00 PM, Steven Blinn wrote: