Adium

"Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more."
I was fed up with having to have ichat and MSN messenger open at the same time and having to switch between the two windows when I was chatting to more than one person. I decided to see if there was an open source solution, and I'm very glad I did. Adium runs flawlessly, looks beautiful and keeps all of my contacts in the same place. Other great features include tabbed chat windows and some really sweet sounds for when people log in and out. I advice anyone who has had the same problem with MSN and ichat to check it out - adiumx.com