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I use tufat's Flashchat on one of my XOOPS sites. It won't display within your theme or anything, it will open in a new chat window, but you can integrate it with XOOPS so that your users will be able to log in with their XOOPS username and password.
This is the link to it, it costs $5 USD i believe
http://www.tufat.com/s_flash_chat_chatroom.htm