From the Tor Blog:
Tor Messenger is a cross-platform chat program that aims to be secure by default and sends all of its traffic over Tor. It supports a wide variety of transport networks, including Jabber (XMPP), IRC, Google Talk, Facebook Chat, Twitter, Yahoo, and others; enables Off-the-Record (OTR) Messaging automatically; and has an easy-to-use graphical user interface localized into multiple languages.
Tor Messenger builds on the networks you are familiar with, so that you can continue communicating in a way your contacts are willing and able to do. This has traditionally been in a client-server model, meaning that your metadata (specifically the relationships between contacts) can be logged by the server. However, your route to the server will be hidden because you are communicating over Tor.
Tor Messenger (Beta) is available Linux, Windows, and OS X.
The complete Tor Blog post has more info and download links.