phpFreeChat

php Free Chat is a simple, fast, and customizable chat server that uses the filesystem for message and nickname storage. It uses AJAX to smoothly refresh and display the chat zone and the nickname zone. It supports customized CSS stylesheets and a plugin system that allows you to write your own storage routines.

Homepage:
http://www.phpfreechat.net
Tar/GZ:
http://downloads.sourceforge.net/phpfreechat/phpfreechat-1.2.tar.gz
Zip:
http://downloads.sourceforge.net/phpfreechat/phpfreechat-1.2.zip
Changelog:
http://www.phpfreechat.net/changelog/1.2
Bug tracker:
http://sourceforge.net/tracker/?group_id=158880&atid=809601
Mailing list archive:
http://sourceforge.net/mail/?group_id=158880
Demo site:
http://www.phpfreechat.net/demo