ICQ is real good but spammy, AOL is the one I am usually on, MSN Messenger, Yahoo Messenger also exist. And to bring them all together is Trillian (you either love it or hate it)
http://www.trillian.cc/
The best one is the one everyone else you know is Online with. Of course that is the point of Trillian - it will log you into all of the ones I mentioned above and will handle IRC (though I havnt used it for that)
Hope it helps.
Rasorow