Most of these programs integrate themselves into AIM and thus aren't required to be running alongside AIM in order to function (instead a menu of options for the program is listed usually in the "My AIM" menu). Common features include ad-removal, tabbed IM windows, logging, etc.
AIM Ad Hack - Removes ads from AIM, allows cloning, and also removes most of the adware that comes with AIM. Suggested if you want to keep AIM's clean look and stick with the basic features
AImutation - Logging, Remove AIM ads, tabbed windows, skinning, etc.
For AIM Classic only
Price: Was several dollars, Now it is free
Deadaim 4.5 - Same thing as Aimutation except it doesn't have skinning.
For AIM Classic only
Price: $5
Deadaim 3.2.1 - Adds functionability to AIM, such as tabbed IM windows and logging.
For AIM Classic only
Price: Free (older version)
GoodAIM (SOCKS proxy) - Includes the same standard features currently in the other AIM addons.
For AIM Classic only
Price: Free
JAMS - a collection of plugins for Triton
Messenger:Mate - Themes, cloning, lock messenger, logging and more.
For AIM Classic only
Price: $10
middle_man - An addon for AIM: logging, adremoval, cloning.
For AIM Classic only
Price: Free
Vario - An addon for AIM: logging, tabbed windows, adremoval, aliasing, etc.
For AIM Classic only
Price: Free