gotcha writes:  

Nuke Live Chat is designed to be a fast and lightweight chat room for your php nuke site. Nuke Live Chat uses ajax to update the messages and users online, making the chat feel fast and responsive without any annoying page reloads or iframes.

You can see Nuke Live Chat in action on www.nukecoder.com but you must be a registered user to post.
Check out the screenshots and download your free copy(registration required) on the Nuke Live Chat download page

Other features include:
Text formating options allow users to choose their own color, size, and style of text.
Smilies panel allows for easy inserting of smilies.
Detachable chat window so users can chat while they browse your site.
An easy to use admin panel includes pagination, sorting, and per page options to manage messages efficiently.