Adium is a free instant messaging application for Mac OS X, released under the GNU GPL

Supported protocols

* AOL Instant Messenger (Including ICQ and .Mac)
* Jabber (Including Google Talk)
* MSN Messenger
* Yahoo! Messenger (Including Yahoo! Japan)
* Bonjour (Compatible with iChat)
* Gadu-Gadu
* Novell Groupwise
* Lotus Sametime

Notable Features

* OTR Encryption
* Tabbed Messaging
* File Transfer
* Webkit Message Display

http://www.adiumx.com/