Mac users check out www.adiumx.com
Nifty little client supports IM over different protocols, all in 1 interface!
Formats supported include AIM, ICQ, MSN, Yahoo!, Rendezvous, Yahoo! Japan, Gadu-Gadu, Jabber, Napster, Lotus Sametime and others.
No need to open different clients to chat with your friends on MSN,ICQ,AIM etc...