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via Roshan Bhattarai's Blog - A blog about PHP,
While talking about chat application, these days people hate those kind application made in PHP which need page refreshing. In this post, I’ll show you six different free
Ajax Chat from http://blueimp.net

A powerful
Ajax- IM

It is not a only a chat application, it is a instant messenger that’s why it is named as Ajax-IM. Very useful for implementing in community based websites.
PHPfreeChat

Another great
Chatr

Another simple
Ajax chat from Ajax-chat.org

Very simple, lightweight and easy to use another
Ajax chat from Blogoscopted

Fancy and avatar based