I am using Yahoo.
Well, I use Y! Messenger only for chatting with my friends.
When I find myself at a time when I can chat and help other marketers and friends, I usually turn to Google Talk for all of my aid. However, some of my uncivilized friends have yet to discover the amazement that the Gmail inbox page has to offer (incorporated video chat box), and therefore I have Adium.
Adium is the equivalent of Pidgin for Linux and Trillian for Windows. It is a multi-stream instant messaging client, that hosts more than the big four messaging circles (MSN, Yahoo, AIM, Google Talk). In addition to these four, I also use Facebook Chat, MySpace IM, ICQ, and several others available through the one program and one contacts window (seamless integration, ladies and gentlemen).
Another application that I have recently joined which does not yet have integration with Adium is Skype, although with several other clients having embedded voice and video chat within Adium (Yahoo, MSN, Google Talk, and others), I believe that Skype will eventually become an addition to the list of messaging clients supported (when I started using Adium last year, only the main four, plus Jabber and GaduGad, were supported).
In addition to this, I've given IRC channels a half-hearted try with Coliquy, but to no avail as far as configuration is concerned. Maybe someday when the need is more necessary. For now, I roll with the long list of contact icons to the right of this post.
I use Meebo. Never heard of it? You have no idea what you're missing.
Technically, Meebo isn't a IM client, but it's a portal. I use it every day.
Technically, I use MSN, AIM, Facebook Chat, and Myspace Chat... all at once.
You do NOT download Meebo. You just go to their website, and log in. You do not have to sign up, but I did because it makes it easier. I sign into my Meebo account and it automatically signs me into all 4 at once.
I believe Meebo does offer their own personal chatrooms, but I just use MSN and AIM.