JWChat aims to be a full featured, web-based Jabber client.
It uses only JavaScript and HTML on the client-side. Currently
it supports basic jabber instant messaging, roster management
and muc-based groupchats.

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

WWW: http://jwchat.sourceforge.net/