GTalk
http://www.google.com/talk/
Okay, I’ll be one of the few people who admit to using GTalk. I mainly use it through Adium which is the Mac client for multi-service chat. I have AIM, GTalk, Y! Messsenger, MyspaceIM, and MSN Messenger all set up through Adium. I just need Skype and the new Facebook IM to finish it off. But I tend to use GTalk or AIM the most.