Apple's Mac OS X comes with a great instant messenger application: iChat. It's a cool little program, but unfortunately, it doesn't support many IM protocols. iChat supports MobileMe, .Mac/MobileMe, AIM, Jabber, and Google Talk.
I found an awesome free program that supports everything iChat does and more. It's called Adium. I use it for 2 AIM accounts at one time and MSN too.
I love that you can have all of your IM accounts in one buddy list so you don't have to keep track of them all like you do in iChat.
Having all of these services in one place can revolutionize the way you interact with them. It's awesome to use Facebook chat without being logged into the website.
Beyond the huge support for chat protocols, Adium gives you fantastic control over the look and feel of your contact list and even how your chat windows look. There are several pre defined themes from which you can choose. Alternatively, you can really easily make your own themes.
Really, it comes down to personal preference, but I recommend Adium to anyone who spends much time using an instant messenging protocol. It's easy. It's fun. And it's free!
You can download it at http://www.adiumx.com
NOTE: Adium's latest version requires OS X 10.4.0 or greater. Users of OS X 10.3.9 can use version 1.0.6 or older. OS X 10.2 users can use Adium 0.89.1 or older.