I have written about ways & tools to combine many different instant messengers into one.
But for most of them you have to download & install a new form of instant chat program that can combine popular chat clients like Yahoo! IM, AOL AIM, MSN & Google Talk.
Meebo is a free multi-chat instant messenger that you can use to combine AIM, Yahoo! Instant Messenger, Google Talk, Jabber, ICQ & MSN chat Ids without the need to download any software.