Up until a day or two ago, on OS X I had always used iChat as my AIM client until a friend of mine recommended something I had heard about but never tried: Adium.
Adium has now stolen my heart, with its highly customizable (and even more minimal-than-iChat) appearance, and exceptionally high tweak factor among other things. In addition to this, Adium is open source, with an extremely active development community. This means that tons of different styles, extras, and plugins are available; in fact there’s a dedicated web page for this because there’s such a large number of them. If you have a Mac, and use AIM (or nearly every other IM protocol), Adium is definitely worth checking out:
