Someone pointed out Fire and I like it. It handles MSN, Yahoo, AOL, ICQ, Jabber and IRC. I am somewhat involved in a effort to get it to support SIMPLE (SIP based) IM as well. It’s a sourceforge.net project.
iChat (built-in) supports only AIM, however it is nicely integrated with Mail and Address Book, showing you online status of your buddies in various places.



