I've never used Flashchat. It's the chat they're using here at vB.org:
https://vborg.vbsupport.ru/chat.php?
It'll log you into the IRC channel with your forum username, set "guest" if the person is unregistered, etc. I've had to pick my way through it since it's not really outlined anywhere how to do it, so I'm sure the coded-by-coders version is a better way to integrate than the way I'm doing it. But since the question "Which chat is used here at vb.org" has been asked about a hundred times and is never answered, I ended up doing it the only way I knew how (webtemplate + EIRC with no java cabs).
The only thing it doesn't do for me right now is outright deny access to the applet to guests, I'm sure it's just an easy conditional, but I haven't gotten around to putting it in yet. It's nice though, my chat is a LOT busier since installing it.