[DeanLag]

I've used Devion's chat box and Cyb's Cyberchat one.
To be honest, Devion's chat box had a better feeling / design but was causing some server overload. Then I switched to Cyb's one and it worked great, easy to use and no template edits.

I do agree that a chatbox can kill some forum activity as in posts and what not but it also does increase more users at one time cause they're all on and chatting together. If the people on my forums have a simple question to ask, they usually do it in the chat box and if they dont get a reply, they take make a thread - which is good cause we don't really like spam =p