Miranda Instant Messenger is a good alternative for those who want multiple instant messaging protocols, but still want to stay small in size. Miranda is around 2.5MB, which is much smaller than Gaim. It does all the major protocols (AIM/ICQ, MSN, Yahoo, etc. It is extensible via plugins. You can download a .zip of the program that is already portable.
http://www.miranda-im.org/release/download.php
-Justin