Well...that depends...do you want the official AIM app, or an IM app that supports AIM as well as others? There are a few out there, and I would be more than happy to direct you to what you would like to use!!
For example...I use AIM and Yahoo, so found an app that supports those and a few others called Slick by Lonely Cat Games. It is very nice on the eyes, and is still in beta, so it is free:
http://www.lonelycatgames.com/?app=slick