AIMSpy is a Mac OS X network monitoring tool written in Objective-C for Cocoa. It will parse all AIM related packets that it finds and display the chat transcripts in an easy-to-use interface.
AIMSpy was written before the AIM protocol was very well documented, and in the years since it has largely ceased to be able to parse AIM packets. One day I'll get around to updating it.
It's old and hasn't been updated as the AIM protocol has changed.