Action Messenger, by Trejkaz Xaoza, is a basic wrapper around Ruby's xmpp4r Jabber library that makes Jabber instant messaging services directly available to Rails applications. (Jabber is an open, XML specification and framework for instant messaging. Google Talk uses it, for example.)

It's still in its infancy and currently only supports basic message sending and receiving of messages, but with instant messaging rapidly merging into the plans of many Web applications, it's a good start. If this sort of functionality is on your wishlist, check it out.