I'm highly impressed with the new Yahoo! Messenger Beta which I stumbled across today. It's got a very nice look, skin-able, and a great integration between the software and your yahoo account. The built in radio function is great, along with the games which is great to waste time at work with.
I used to think the MSN Messenger was the best instant messaging client out there, but now I'm all Yahoo! Messenger. It's so slick. Give it a shot, and message "jezzshields". I really think you'll enjoy it, and it's much slicker looking then MSN. You can even set up a contact (or import from Outlook) into Yahoo, to get it all hooked together. If anyone uses this, please message me! I only have 9 people on my list. You'll notice it has a much more prominent position in my contact information.