Kopete
Having used Gaim for years on my Linux machine I have recently discovered a superior IM client for Linux named Kopete. It is a KDE based application (but don’t let that put you off :p it works under GNOME too).
It is a lot more slick than GAIM which to be honest I have always found a little clunky. The interface is small which is exactly what I want from IM and further more it is one of the most intuitive applications I have ever used.