There's always FlashChat
FlashChat - Customizable Flash Chat Room and Live Support system for your PHP/MySQL-enabled website. Try it now!
If your on a shared hosting program, I'd check with your hosting provider to see if they allow for 3rd party chat software to be installed. Some webhosts (at least one of my old ones) did not like these programs being added and had no problems freezing any account using additional resources which they attributed to the chat software.
It may not be an issue but it never hurts to check it out just to make sure.