Trillian
I have contacts with AOL, Yahoo, and MSN so Trillian is really necessary for me. Trillian also is VERY resource friendly when compared to most IM clients, even Pidgin. It never freezes or slows my computer down and it has a very clean interface free of the ads and stupid stuff that fill up the major IM clients. Also, it starts up cleanly and in the task tray, Trillian is usually open and hidden on my computer before I even know it.
If I am at a school computer or a computer that I obviously ca't install things onto, I have a Meebo account which is very smilar to trillian but is online based so no software installation needed.