Adium
I’ve used lots of different instant-messaging clients, but I must say that I’ve never been as impressed by any of them as quickly as I’ve been impressed by Adium.
Adium is a free AIM client, written by Adam Iser. It’s lean, small, and very fast, which are reasons enough for it to replace my default OS X messenger client. The installation was (as always, on OS X) drag’n’drop simple, and the configuration options are very thorough. My install takes up a total of 2.5mb of drivespace, and in its idle state the app uses roughly 0% CPU load. If you’re interested in how it works, the author has released the program under the GPL, and has the source code freely-available on his site.