It seems like it might be something pretty easy to build yourself. When I first learned what AJAX was, several years ago, my first "let me try this out" project was a chat room. A table of messages, a table of users and a table of sessions linking users to messages. Then a simple form and some XHR checks every few seconds to look for new messages.
The benefit of rolling your own would be a highly customized design that is truly integrated into your site. Plus you could easily make it do other things like send your client an SMS message when a user attempts to start a chat session.