Don't use any. It's extremely rare that I will IM someone.. Don't really like that form of communication - too slow and bulky. Don't have that kind of patience.. If I need to talk with someone, I call them - or will email. Guess I'm not as much a Geek as "the test" says I am eh?
MSN, AIM, YIM (because there are loads of people who will not accept that it sucks), and ICQ though trillian. Then IRC and Xfire.