Digsby is a good client, which I believe is based off of libpurple (the same engine behind Pidgin). It supports the major (and most minor) chat protocols, including the standard group of YahooIM, AIM, MSN Messenger, GChat, Jabber, ICQ, IRC, and Facebook.
Its free as in beer, but not free as in speech, and has more than a few ads (EDIT: understatement). I stopped using it while it was still in beta after they released a build with a memory leak bug (500MB is too much for a chat client), but that was 2 years ago.
MASSIVE VERY IMPORTANT EDIT: LifeHacker did some digging and posted a rant on how Digsby does all sorts of nastiness (everything from useless advertising "options" to stealing CPU cycles). I can't recommend Digsby any longer, and would heartily recommend you do not use Digsby.