pas trop simple à installer lol phpfreechat
http://www.phpfreechat.net/download
fonctionne sans mysql, uniquement avec ajax et php, il en existe plusieurs versions, j'utilise les 2

très simple à installer:
GroChat is a light weight web based chat software coded with PHP which doesn't need a database to work. It was developed to offer a light weight solution to people who want to install a web based chat software without having to install / customize a dat
don fonctionne sans mysql (ouf)
http://sourceforge.net/project/showfiles.php?group_id=130502