Adium
Adium is a free and open source instant messaging application for Mac OS X. Adium allows you to chat with all your friends using different chat services at the same time. Among others, it supports: AOL Instant Messenger, including ICQ and MobileMe, MSN Messenger, Jabber, including Google Talk and LiveJournal, Yahoo! Messenger, including Yahoo! Japan, and Bonjour. It is also compatible with iChat, MySpace IM, Facebook Chat, Gadu-Gadu and more.