Adium is probably the most visually customizable chat client I have ever seen. From themes and backgrounds to themes and dynamic Dock icons, Adium does it all.
One of my favorite customizations in Adium is the ability to adjust the location of the tab bar. It can be placed on top, at the bottom, or on either side of a chat window.
To adjust the placement of the tab bar, simply enter Adium’s preferences and choose the General pane. Where it says “Show tabs on the:,” select which side you want the bar placed, and you’re set!