AOL hasn't updated their Mac client in years, so why must we keep looking at their blinking ads? This is a bit of a hack, but a rather innocuous one. You see, each time you launch AIM (which should be rare; but sadly there are still features that only work in the official client), it establishes a network connection to annoying-ad-central and picks something nice and obnoxious for you to enjoy.
But with Objective Development's Little Snitch ($25 shareware), you can actually prevent your client from ever communicating with the ad server. Instead, all your client will be able to display is a tame AOL logo. When I tested it, I simply allowed connections while signing-on, and denied connections after my buddy list appeared. The client was fully functional, sans ads! Perhaps someone can tell us precisely which servers to block...
[robg adds: I'm not sure what the chances are that AOL will update AIM for the Mac, but if you're blocking their ads, there's a chance you're directly impacting those odds -- lower ad revenue means less incentive to create a newer version. This assumes, of course, that AOL has a way to track ad revenue back by client version, and that they do so...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050901014049789