Trillian 2.013 with the kimae skin is really simple, clean, and easy on the system.
Totally black, has all the buttons hidden, but easily accessable by right clicking on the chat window.
I've been using this skinned version of trillian 2.013 for a very long time, and it feels so alien to use anything else.
the latest version (3.1) is all fancy and feature packed, too bloated for my tastes.