HatTip to Sam. I just recently came across Adium, which is a third-party instant messaging application for Mac OS X users. It's been around for a while, so I assume some of you Mac users already know about this.
I found this app convenient since it allows you to access your multiple IM accounts (e.g. AIM, MSN, Jabber, Yahoo, Facebook, etc.). It's Trillian or Meebo for your Mac desktop.