Adium is a fast and free instant messaging client which supports AIM, ICQ, Jabber, MSN, Yahoo!, Google Talk, Yahoo! Japan, Bonjour, Gadu-Gadu, QQ, Novell Groupwise, SIP/SIMPLE (Text), and Lotus Sametime. Adium supports beautiful WebKit message display, tabbed messaging, encrypted chat, file transfer, and more. Give it a try; you won't look back.
For additional information about this software, please see this GROK article: Adium: Overview