ARSC Really Simple Chat
A webchat software written in PHP and MySQL. It is compatible with every browser known today, even text-based ones. [Open source, GPL]
BlaB! Lite
(formerly SimpleChat!) Fast, reliable, XHTML compatible web chat written in PHP and JavaScript.
Chatty
A nice script that lets create a simple chat in a few minutes. Requires MySQL. [Open source, GPL]
Open Chat
Features include unlimited and private channels; moderators, whisper mode, timed sessions and multiple language support.
WebChat
A chat application that features multiple rooms, no browser refresh, database back-end, emotions, private messages, and multiple language support.
Webfroot Shoutbox
Allows users to post little blurbs on your website like a small guestbook/chat box. Inline or IFRAME, powered by mySQL.
X7 Chat
A PHP chat script with unlimited users, rooms, and messages, IRC style commands, full message formating. [Freeware]