Open Source & Facebook-Like Chat Client: Converse.js
Converse.js is an open source chat application that can connect to any XMPP/Jabber server (like Google Talk,Skype, etc.).
It sits in the footer of web pages just like Facebook-chat and has support for one-to-one or multi-user conversations.
Contact requests can be accepted/declined, statuses can be set (online, away..) and there are “typing” notifications to inform the “listening” users.
With a little custom coding, single-site login can be accomplished (users logged in to the site will automatically be logged in to the chat).