Adium is a free and open source instant messaging application for Mac OS X, written using Mac OS X's Cocoa API, released under the GNU GPL and developed by the Adium team. Based on the libpurple protocol library, Adium can connect you to any number of messaging accounts on any combination of supported messaging services and then chat with other people using those services.

Features:

• partial address-book integration
• cool-looking tabs
• multiple protocols for instant messaging
• compact contact list
• support for AIM, Yahoo, MSN, Jabber
• Bonjour compatible with iChat, Facebook Chat, and Gadu-Gadu
• unlimited accounts
• as many names on as many services as you want
• draggable tabs
• any number of tabs in any number of windows
• highly customizable yet intuitive preferences
• address-book integration
• iTunes and other AppleScript substitution buddy icons and away messages in AIM

Requirements:
OS X 10.7+
64-bit Intel