Drupal site Instant Messaging
Other modules that provides chat room are


DrupalChat does not require an additional server component/server. This ease installation but can become a performance issue as PHP/Apahce are not a good fit to efficiently server chat-like application. If scalability is an issue, it supports a more robust solution based on Ape's Ajax Push Engine. The module description implies that the module allow other backend to be written.
Drupal Strophe provides XMPP chat integration in Drupal. It only supports the ejabberd server.
Chat Room doesnt require an additional server component/server. The author is working on a Node.JS integration module aimed to be used by the D7 version of Chat Room to solve the scalability issue.