I would recomend AIM+. You can get it at http://www.big-o-software.com . Here are some features listed at the website:

• Automatic (but optional) logging of all your IMs (even chat rooms!), creating a history which you can browse using the AIM+ History Viewer (part of AIM+).
• Ad-Removal – say goodbye to all those pesky ads that clutter up your Buddy List!
• Customizable Buddy List Window – you can choose which components of the Buddy List you want to be shown, in any combination you desire, including the menus, the Buddy List tabs, the chat buttons, and the general buttons.
• Cloning – you can run multiple instances of AIM to sign on with as many different screennames as you want! <
  

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  THIS IS WHAT YOU WANT
• Customizable hotkey to pop-up/hide your Buddy List.
• Adjust the transparency of your Buddy List or IM/Chat windows!
• Easy installation and automatic uninstallation. (leaves no trace in the registry)
• Flawless & smooth upgrading to new versions (simply install the new version) – you won’t lose any settings.
• Windows XP-style 32-bit icons
• Seamless integration with AIM (using AIM’s System Menu)

(Edited by Jeff – Added bullets to make it easier to read}