Empathy
My favorite application for instant messaging - pidgin (previously gaim) seems to be taking up too much of resources on a desktop with decent hardware that I wouldn't want to upgrade or throw away anytime soon. On my machine, since instant messenger is something that stays on all the time, I'd prefer an IM client that occupies less of the available resources and above all, is light weighted and less complicated for use.
On Ravi's lead, I installed empathy today and tried it out. At first glance, it looked impressive.