Adium is a free all-in-one instant messenging application for Mac OSX computers which supports simultaneous connections to the following IM services:

AOL
ICQ
.MAC
Google Talk
Windows Live MSN
Jabber
Yahoo!
LiveJournal
Novell Groupwise

Among the features that Adium supports include:

OTR Encryption: Off-the-Record (OTR) Messaging allows you to have private conversations over instant messaging by providing Encryption, Authentication and Anti-Spoofing
Tabbed windows: Supports multiple chat windows in a single window
File Transfer Send and recieve files ober IM
Webkit Message Display: Change apperance and themes with extras (iPhone theme)

Download Adium here