JWChat

JWChat is a full featured, web-based Jabber client. Written using AJAX technology it relies on JavaScript and HTML at the client-side only. It supports basicm jabber instant messaging, roster management and group chats based on the MUC protocol.

JWChat is an advanced instant messenger (IM) just like AIM, MSN Messenger, Yahoo! Messenger or ICQ. This means you can manage your contacts, communicate with other users in 1:1 chat sessions or join a group chat room for collaborative communication. Unlike other IMs you can use this with your web browser without having to install any additional software at all.

http://blog.jwchat.org/jwchat/