There are a bunch.
phpMyChat is an open source free app
which is active even this year of 2010 with new releases and this looks good.
Latest release was only like 2 weeks ago.
phpMyChat can use MySQL plus several other databases for storage.
http://sourceforge.net/projects/phpmychat/
Besides, 37 languages is really impressive !!
and shows there are probably many users all over the world.
This is good when you need to ask things.
phpMyChat is an easy-to-install, easy-to-use multi-room PHP/DB chat.
It is currently available for MySQL, PostgreSQL, and ODBC, and the work on Oracle is in progress.
It supports IRC-like commands, moderators, and is available with 37 languages.