Well, there's three. There's Oscar (AIM/ICQ), there's MSN Yahoo, and then there's Jabber (also accessible from Google Talk). Of these three, only Jabber is on it's way to becoming IETF standard. Jabber has a couple million more users than Yahoo these days.
If you're looking for IM unification, the future is now:
Give Jabber a spin, and tell your friends.
If you're on Google Mail, your email address is also your Jabber ID, and most clients (with the notable exception of Trillian) support Jabber fairly well.